OUTLINES  OF  PHYSIOLOGY 


JONES  AND  BUNCE 


Cranium. 


7  Cervical  Vertebrae. 

Clavicle. 
Scapula. 


Humerus. 


Ilium. 

Ulna. 
Radius. 

Pelvis. 


Bones    of   the   Carpus. 
Bones  of  the  Meta- 
carpus. 
Phalanges  of  Fingers. 


Fatella. 


Tibia. 
Fibula. 


Bones  of  the  Tarsus. 
Bones  of  the  Metatarsus. 
Phalanges  of  Toes. 


THE  SKELETON  (AFTER  HOLDEN). 


OUTLINES 


OF 


PHYSIOLOGY 


BY 

EDWARD  GROVES  JONES,  A.B.,  M.D.,  F.A.C.S. 

PROFESSOR  OF  SURGERY,  EMORY  UNIVERSITY 
(ATLANTA  MEDICAL  COLLEGE) 

AND 

ALLEN  H.  BUNCE,  A.B.,  M.D. 

ASSOCIATE  IN    MEDICINE,   EMORY  UNIVERSITY 
(ATLANTA  MEDICAL  COLLEGE) 


FOURTH  EDITION,  REVISED 
in  ILLUSTRATIONS 


PHILADELPHIA 

P.  BLAKISTON'S   SON   &  CO. 

1012    WALNUT    STREET 


,  •    *   -:  c     ^  •     J 

>,»          -  •    e  -•  •-  *  s 


COPYRIGHT,  1916,  BY  P.  BLAKISTON^S  SON  &  Co. 


TO 
DOCTOR  WILLIAM  S.  KENDR1CK 

SENIOR  PROFESSOR  OF  MEDICINE,  EMORY  UNIVERSITY 
(ATLANTA  MEDICAL  COLLEGE) 

THESE  PAGES  ARE  AFFECTIONATELY  DEDICATED 


'Oi^v-41  r~***j 

o7157 


PREFACE  TO  FOURTH  EDITION 


IN  preparing  this  revision  the  majority  of  changes  have 
been  of  details,  the  chapters  and  general  arrangement  of  the 
book  have  been  kept  as  in  the  third  edition.  A  number  of 
new  illustrations  have  been  added  and  others  have  been  re- 
engraved.  Every  effort  has  been  made  to  bring  the  subject 
matter  up  to  date  and  to  keep  the  book  up  to  the  highest 
standard  in  every  way. 

We  appreciate  the  reception  which  has  been  accorded  the 
previous  editions  and  hope  that  this  will  prove  even  more 
valuable  to  the  student  and  practitioner  than  those  which 

have  preceded  it. 

EDWARD  G.  JONES. 

ALLEN  H.  BUNCB. 
ATLANTA,  GA. 


vn 


PREFACE  TO  FIRST  EDITION 


THIS  volume  has  been  prepared  with  the  view  of  present- 
ing, in  as  convenient  form  as  possible,  the  essential  facts  of 
modern  physiology  as  related  to  the  practice  of  medicine.  In 
the  execution  of  this  purpose  brevity  has  been  made  a  prime 
consideration;  therefore,  such  details  as  are  of  secondary 
importance  are  omitted,  theories  are  avoided,  and  conclusions 
are  recorded  without  argument.  There  is  no  short  road  to 
knowledge,  and  it  would  be  unfortunate  should  such  a  book 
as  this  in  any  way  discourage  extended  research;  but  stu- 
dents in  college  have  none  too  much  time  to  devote  to  any  one 
subject,  and  any  simple  collection  of  pertinent  facts,  however 
brief,  can,  if  reliable,  be  used  to  great  advantage.  I  have  en- 
deavored, however,  to  make  the  work  sufficiently  exhaustive 
to  be  self-explanatory,  believing  that  otherwise  economy  of 
expression  is  practised  at  the  expense  of  the  reader's  interest. 

A  maximum  of  space  has  been  given  to  those  subjects 
which  seem  of  most  practical  importance.  The  chemistry  of 
the  body,  the  special  senses  and  embryology  have  not  been 
treated  in  great  detail.  It  has  been  thought  undesirable  to 
omit  a  brief  anatomical  description  of  the  separate  organs 
discussed. 

In  the  preparation  of  this  volume  no  claim  to  original  in- 
vestigation is  made.  The  writings  of  various  authorities 
have  been  freely  drawn  upon.  Especial  acknowledgment 
is  due  to  the  following  authors:  Howell  (American  Text- 
book), Halliburton  (Kirkes5  Handbook),  Flint,  Verworn 
and  Stewart. 

I  am  under  obligations  to  Dr.  J.  Clarence  Johnson,  whose 
lectures  have  been  of  great  value  to  me,  and  to  Dr.  Frank  K. 
Boland,  who  has  written  the  whole  of  Chapter  II.,  read  the 
proof  sheets,  and  rendered  other  valuable  assistance  in  con- 
nection with  the  work.  E.  G.  J. 

ATLANTA,  GA. 

ix 


CONTENTS 


PAGE 

INTRODUCTION xv 

CHAPTER  I. 

THE    CELL -     .     .  i 

CHAPTER  II. 

THE  ELEMENTARY  TISSUES    .     . 7 

The    epithelial    tissues 7 

The  connective  tissues n 

The    muscular    tissues 19 

The  nervous  tissues 22 

CHAPTER  III. 

PHYSIOLOGICAL  CHARACTERISTICS  OF  MUSCLE 23 

CHAPTER  IV. 

SECRETION 27 

Sebaceous      glands 3° 

Mammary  glands •  31 

Thyroid     gland 32 

Adrenal    glands 33 

Pituitary  body 34 

Testis  and  ovary 34 

CHAPTER  V. 

THE   BLOOD 35 

CHAPTER  VI. 

THE  CIRCULATION  OF  THE  BLOOD 41 

The   heart    . .  42 

Circulation    in    Wood- vessels    .     .     .     .     .     ....     .     .     .  46 

xi 


Xll  CONTENTS 

PAGE 

Structure  of  the  blood-vessels 47 

The  lymph 57 

CHAPTER  VII. 

THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 63 

Foods 63 

Digestion '  .  68 

Prehension 70 

Digestion  in  the  mouth 71 

The  salivary  glands  and  their  secretion 71 

Deglutition 78 

Digestion  and  absorption  in  the  stomach 81 

The   gastric   glands 85 

Digestion  and  absorption  in  the  intestines 96 

The  small  intestine 96 

The  large  intestine 117 

Absorption    in    general 122 

Absorption  from  the  alimentary  canal 126 

CHAPTER  VIII. 

RESPIRATION 131 

Anatomy  of  the   respiratory  organs 132 

Mechanism  of   respiration 139 

CHAPTER  IX. 

NUTRITION,  DIETETICS  AND  ANIMAL  HEAT 171 

Nutrition 171 

Dietetics 181 

Animal    heat 184 

CHAPTER  X. 

EXCRETION 192 

The    kidneys 192 

The    skin 208 

CHAPTER  XL 

THE    NERVOUS    SYSTEM 214 

The  cerebro-spinal  axis 235 


CONTENTS  Xlll 

PAGE 

The   spinal   cord 238 

The    encephalon 251 

The    medulla    oblongata 251 

The  pons  varolii 255 

The   crura    cerebri 256 

The     cerebrum 260 

The   cerebellum 274 

The   cranial    nerves 276 

The    spinal    nerves 296 

The  sympathetic  system 298 

CHAPTER  XII. 

THE    SENSES 305 

Common   sensations 305 

Special     sensations 306 

The  sense  of  touch 306 

The  sense  of  smell 307 

The  sense  of  sight 308 

The  sense  of  taste   .     .     .     .' 317 

The  sense  of  hearing 318 

The  production  of  the  voice 325 

CHAPTER  XII F. 

REPRODUCTION 328 

INDEX 365 


INTRODUCTION 


THE  science  which  treats  of  the  structure,  function  and 
organization  of  living  forms,  both  vegetable  and  animal,  is 
called  biology.  That  branch  of  biology  which  describes  ani- 
mal life  exclusively  is  termed  zoology,  while  that  branch 
which  describes  vegetable  life  exclusively  is  termed  botany. 

The  study  of  the  form  of  organisms,  both  vegetable  and 
animal,  is  termed  morphology.  Morphology  is  further  di- 
vided into  ( i )  histology,  which  treats  of  the  formed  elemen- 
tary constituents  of  organisms,  and  (2)  anatomy,  which 
treats  of  the  parts  and  organs  of  the  organism. 

After  the  form  and  structure  of  an  organism  has  been 
studied,  the  next  step  is  the  study  of  the  work  which  the  or- 
ganism has  to  perform.  This  study  of  the  vital  phenomena, 
or  life,  of  the  organism  is  called  physiology.  Physiology 
may  be  either  animal  or  vegetable.  Human  physiology  is 
that  branch  of  physiology  which  treats  of  the  vital  phenom- 
ena occurring  in  man. 

The  structural  unit  of  the  body  is  the  cell.  Myriads  of 
cells  are  grouped  together  to  form  organs.  An  organ  may  be 
defined  as  a  group  of  cells  combined  together  to  perform 
some  special  function,  e.  g.,  the  kidney  is  an  organ  whose 
special  function  is  the  secretion  of  urine.  The  organs  are 
further  grouped  together  to  form  systems.  Thus  we  have 
the  circulatory  system  composed  of  the  heart,  arteries,  veins, 
and  capillaries.  Now  the  study  of  the  function  which  the 
circulatory  system  has  to  perform  is  the  physiology  of  circu- 
lation. Likewise  we  may  subdivide  physiology  into  the  phy- 

xv 


XVI  INTRODUCTION 

siology  of  the  nervous  system,  the  physiology  of  the  digestive 
system,  the  physiology  of  the  respiratory  system,  etc. 

Thus  we  have  seen  the  relation  physiology  bears  to  the 
other  sciences  dealing  with  life  and  the  special  part  human 
physiology  plays  in  the  whole  physiology.  .  In  the  following 
pages  we  have  endeavored  to  give  a  brief  outline  of  human 
physiology. 


OUTLINES  OF  PHYSIOLOGY 


CHAPTER  I./'* 


THE  CELL. 


ALL  the  tissues  of  the  body  are  made  up  of  cells  and  inter- 
cellular substance.  All  the  cells  are  descended  from  one 
parent  cell,  called  the  ovum,  while  the  intercellular  substance 
is  created  through  the  medium  of  the  cells. 


Nuclear 
membrane. 


Linin.    .,. 


Nuclear  fluid 
(matrix). 


Nucleolus. 


Chromatin  cords 

(nuclear 

network). 


Nodal  enlarge- 
ments of  the 
chromatin. 


Cell  membrane. 
~\  -  ----•  Exoplasm. 

Microsomes. 
Centrosome. 

-*   Spongioplasm. 

y Hyaloplasm. 

--i»—-    Foreign  inclosures. 


FIG.  i. — Diagram  of  a  cell. 

Microsomes  and  spongioplasm  are  only  partly  drawn.     (Brubaker.) 

A  cell,  which  is  the  histologic  unit  of  the  body,  may  be  de- 
nned as  an  irregular  round  or  oval  mass  of  protoplasm  of  mi- 
croscopic size,  enclosing  a  small  indistinct  spherical  body,  the 
nucleus. 


2  THE    CELL 

The  essential  parts  of  the  cell  are,  (i)  the  cytoplasm, 
which  is,  a  special  name  given  to  the  protoplasm  forming  the 
cell-body  and  (2)  the  nucleus,  which  is  a  small  round  or 
oval  body  embedded  in  the  cytoplasm.  A  great  many  cells 
are  surrounded  by-  a  cell-wall  or  cell-membrane,  but  this 
c;aaiftot  be.  regarded ;as  one  of  the  essential  elements  since  all 
cells  do  not  possess  such  membranes. 

-,*(i)'  The  Cytoplasm. — This  is  a  gelatinous  or  semi-fluid, 
granular  substance,  transparent  and  generally  colorless. 
Chemically  it  consists  of  water  and  salts,  together  with  vari- 
ous organic  substances,  called  proteids,  which  are  complex 
combinations  of  carbon,  hydrogen,  oxygen  and  nitrogen,  and 
sometimes  phosphorus  and  sulphur.  The  proteids  of  the  cy- 
toplasm contain  little  phosphorus,  while  those  of  the  nucleus 
are  rich  in  it. 

The  cytoplasm  does  not  always  present  the  same  structural 
appearance  since  its  constituents  vary  in  their  condition  and 
arrangement.  In  some  cells  it  has  a  clear  homogeneous  ap- 
pearance, while  in  others  it  contains  fine  spherical  particles 
which  give  it  a  granular  structure.  When  these  granules  are 
large  and  clear,  and  are  surrounded  by  denser  areas  they  give 
to  the  cytoplasm  an  alveolar  outline.  But  most  frequently 
the  cytoplasm  contains  in  its  structure  a  mesh  work  of  threads 
or  fibrils  which  give  it  a  reticular  appearance.  This  network 
of  fibrils  is  called  the  spongioplasm  which  encloses  a  less  firm 
portion,  the  hyaloplasm  ( Fig.  i ) . 

However,,  in  all  these  varieties,  the  cytoplasm  has  both  an 
active  and  a  passive  structure.  In  young  granular  cells  the 
active  substance  is  represented  by  small  spherical  particles, 
called  microsomes  (Fig.  i).  These  are  not  always  evenly  dis- 
tributed throughout  the  cytoplasm,  but  are  grouped  in  an  area 
near  the  nucleus,  while  the  area  next  the  cell-wall  is  almost 
free  from  granules.  The  dense  inner  area  is  called  the  endo- 
plasm,  while  the  clear  outer  area  is  called  the  exoplasm 

(Fig.  i). 

(2)  The  nucleus,  which  is  the  second  essential  part  of  a 


THE  VITAL  PHENOMENA  OF  CELLS  3 

typical  cell,  is  a  small  round  or  oval  body  contained  within 
the  cell.  It  is  usually  surrounded  by  a  distinct  nuclear  mem- 
brane, except  during  division.  Nuclei  play  an  important  role 
in  cell  reproduction  and  cell  nutrition.  They  are  character- 
ized by  their  affinity  for  certain  stains,  e.  g.}  hematoxylin. 

The  substance  of  the  nucleus,  the  karyoplasm,  may  be  di- 
vided into  two  parts — the  nuclear  fibrils  which  form  an  ir- 
regular reticulum,  and  the  nuclear  matrix  which  forms  the 
intervening  semi-fluid  mass.  The  nuclear  fibrils,  when  prop- 
erly stained,  are  found  to  consist  of  minute  irregular  masses 
of  a  deeply  colored  substance,  called  chromatin,  in  recogni- 
tion of  their  affinity  for  certain  stains.  The  chromatin  par- 
ticles are  supported  within  delicate  and  colorless  threads  of 
linin.  The  nuclear  matrix,  which  is  semi-fluid  in  character 
and  which  occupies  the  spaces  between  the  nuclear  fibrils, 
possesses  a  very  weak  affinity  for  the  stains  used  to  color  the 
chromatin.  Hence,  it  usually  appears  clear  and  untinted. 
Chemically,  the  chromatin  contains  a  substance  nuclein, 
which  is  rich  in  phosphorus. 

The  nucleolus  ordinarily  appears  as  a  small  spherical  mass 
among  the  nuclear  fibrils.  It  is  supposed  to  be  of  little  sig- 
nificance in  so  far  as  the  vital  phenomena  of  the  cell  are  con- 
cerned. 

The  Vital  Phenomena  of  Cells. — The  vital  phenomena  of 
the  cell  include  all  those  processes  and  changes  which  it  un- 
dergoes during  its  life  and  which  take  place  in  the  perform- 
ance of  its  various  functions.  They  include  (i)  metabolism, 
(2)  growth,  (3)  reproduction  and  (4)  irritability. 

(i)  Metabolism,  includes  all  those  processes  by  which  the 
cell  is  enabled  to  select  from  the  various  substances  furnished 
it  and  convert  them  into  its  own  substance,  and,  secondly, 
those  processes  whereby  the  cell  is  enabled  to  cast  off  the 
waste  products  set  free  by  its  activity.  The  first  process,  that 
by  which  the  cell  takes  the  simple  substances  furnished  it  and 
converts  them  into  its  .complex  compounds,  is  called  anabol- 
ism,  or  constructive  metabolism. 


4  THE  CELL 

The  second  process,  that  by  which  the  cell  breaks  up  these 
complex  compounds  formed  by  anabolism  and  discharges 
them  from  its  substance,  is  called  katabolism,  or  destructive 
metabolism.  A  good  example  of  anabolism  is  that  by  which 
the  vegetable  cells  take  such  substances  as  carbon  dioxide, 
water  and  inorganic  salts  and  prepare  food-material  for  the 
nutritive  and  katabolic  processes  in  animals. 

(2)  Growth   is   the   natural   sequence   of   the   nutritive 
changes  effected  by  metabolism  and  may  be  unrestricted  and 
equal  in  all  directions.    However,  this  is  not  usually  the  case 
as  is  shown  by  the  fact  that  cells  are  so  intimately  associated 
with  other  structural  elements  as  to  influence  and  modify 
their  growth.     These  result  in  unequal  growth,  to  which 
the  specialization  of  cells  is  due.    Examples  of  the  unequal 
growth  of  cells  are  shown  in  the  columnar  cells  of  epithe- 
lium, the  neurones  of  nervous  tissue,  the  fibers  of  muscle 
tissue,  etc. 

(3)  Reproduction  may  be  regarded  as  the  culmination  of 
the  activities  of  the  cell,  for  by  this  process  the  cell  loses  its 
individuality  and  continues  its  life  in  that  of  its  offsprings. 
There  are  two  methods  by  which  cells  may  reproduce  them- 
selves,  (i)   by  direct  cell  division  or  amitosis  and   (2)   by 
indirect  cell  division  or  mitosis. 

(4)  Irritability  is  that  property  of  cells  whereby  they  are 
enabled  to  respond  to  stimuli,  i.  e.,  to  change  their  form  and 
shape  in  response  to  these*  stimuli.     The  various  stimuli 
which  affect  cells   may  be  mechanical,   thermal,   nervous, 
chemical  or  electrical. 

Cell  Division. — (i)  Direct  cell  division,  or  amitosis,  is  the 
simplest  form  of  cell  division.  In  this  form  of  cell  division 
the  nucleus  and  protoplasm  constrict  in  the  middle  until  two 
new  cells  are  formed.  This  form  of  cell  division  does  not 
occur  in  the  higher  animals  except  as  a  secondary  process. 

(2)  In  the  higher  animals  cell  division  takes  place  chiefly 
by  the  indirect  or  mitotic  method.  This  may  be  described 


CELL  DIVISION 


briefly  as  follows :  In  the  beginning  of  this  phenomenon,  the 
nucleus,  which  plays  the  most  important  role,  grows  larger. 
Its  chromatin  greatly  increases,  becomes  contorted  so  as 
to  form  a  dense  convolution,  the  close  skein,  or  spireme. 
Then  the  chromatin  fibrils  further  thicken,  become  less  con- 
voluted and  form  irregularly  arranged  loops,  the  loose  skein. 


Close    Skein 

(viewed  from  the 

side);    Polar    field. 


Loose  Skein 

(viewed  from  above — i.  e.,  from 
the  pole). 


Mother  Stars 

viewed  from 

the  side). 


Mother  Star          Daughter  Star 
(viewed  from  above). 


Beginning.  Completed. 

Division  of  the  Protoplasm. 


FIG.  2. — Karyokinetiq  figures  observed  in  the  epithelium  of  the  oral 
cavity  of  a  salamander. 

The  picture  in  the  upper  right-hand  corner  is  from  a  section  though  a  dividing 
egg  of  Siredon  pisciformis.  Neither  the  centrosomes  nor  the  first  stages  of  the 
development  of  the  spindle  can  be  seen  by  this  magnification.  X  560.  (From 
Brubaker.) 

During  the  formation  of  these  skeins  the  nuclear  membrane 
and  the  nucleoli  disappear.  The  fibrils  of  the  loose  skein  now 
separate  at  their  peripheral  turns  into  a  score  of  loops,  the 
closed  ends  of  which  point  toward  a  common  center — a  clear 
space  called  the  polar  Held.  When  seen  from  above  these 
loops  of  chromatin  make  a  wreath,  called  the  mother  wreath; 


O  THE  CELL 

when  seen  from  the  side,  they  make  a  star,  called  the  mother 
star  or  aster.  While  the  loose  skeins  are  forming,  delicate 
striae  appear  within  the  achromatin,  so  disposed  as  to  make 
their  bases  within  the  polar  field  and  directed  toward  one 
another,  and  their  apices  directed  toward  the  future  new  nu- 
clei. These  achromatin  figures  constitute  the  nuclear 
spindle.  They  then  arrange  themselves  into  two  daughter 
wreaths,  or  asters,  similar  to  the  mother  star.  At  this  junc- 
ture the  cell  protoplasm  begins  to  divide  by  becoming  con- 
stricted in  the  center.  The  daughter  stars  are  converted  into 
two  new  nuclei,  in  the  inverse  order  to  that  by  which  the 
original  nucleus  was  broken  up.  Nuclear  membranes  and 
nucleoli  appear,  the  cell  protoplasm  divides  into  two  new 
cells,  and  the  cycle  is  completed. 

Derivation  of  Tissues. — The  primary  parent  cell  divides 
into  an  innumerable  mass  of  cells  which  is  called  the  blasto- 
derm. The  blastoderm  soon  divides  into  two  more  or  less 
distinct  layers,  an  outer  and  an  inner,  named  ectoderm  and 
entoderm,  between  which  a  middle  layer  develops,  the  meso- 
derm.  From  these  three  primary  layers  all  of  the  various 
tissues  of  the  body  are  later  developed.  (See  Embryology.) 


CHAPTER  II. 
THE  ELEMENTARY  TISSUES. 

THE  tissues  which  make  up  the  various  organs  and  parts 
of  the  body  may  be  divided  into  the  following  groups:  (i) 
Epithelial  tissue,  (2)  connective  tissue,  (3)  muscular  tissue 
and  (4)  nervous  tissue. 

The  Epithelial  Tissues. 

The  epithelial  tissues  include  those  which  form  the  cover- 
ing for  the  body,  the  lining  of  the  digestive  canal,  the  respira- 
tory tract  and  the  genito-urinary  tract.  They  also  constitute 
the  derivatives  of  the  epidermis,  such  as  nails,  hair,  seba- 
ceous glands,  and  the  lining  of  the  glands  connected  with  the 
digestive  and  genito-urinary  systems. 

These  tissues  are  composed  of  cellular  and  intercellular 
elements  and  perform  various  functions  in  different  parts  of 
the  body.  In  the  skin  where  they  constitute  the  epidermis, 
they  protect  the  delicate  surface  of  the  true  skin  beneath ;  in 
the  alimentary  and  genito-urinary  canals  they  aid  in  secre- 
tion and  excretion ;  in  the  respiratory  system  they  preserve 
an  equable  temperature,  while  in  all  internal  parts  they  yield 
lubricants. 

These  tissues  are  characterized  by  the  preponderance  of 
the  cellular  over  the  intercellular  elements.  The  intercellular 
structure  consists  of  a  cement  substance  which  holds  the  cells 
together  and  through  which  the  food  for  the  cells  is  ab- 
sorbed. They  contain  no  blood-vessels  and  no  nerves.  The 
tissue  usually  rests  upon  a  basement  membrane,  or  membrana 
propria,  which  is  a  modification  of  the  connective  tissue  be- 
neath. 


8  THE  ELEMENTARY  TISSUES 

Varieties. — The  varieties  of  epithelium  may  be  classified 
as  follows:  (i)  Squamous,  (a)  simple,  consisting  of  a  single 
layer,  (b)  stratified,  consisting  of  several  layers;  (2)  Col- 
umnar, (a)  simple,  (b)  stratified;  (3)  Modified,  (a)  ciliated, 
(b)  goblet,  (c)  pigmented,  (d)  glandular,  (e)  neuro-epi- 
t  helium. 

(i)  Squamous  Epithelium. — {a)  Simple  squamous  epithe- 
lium consists  of  a  single  layer  of  cells  which,  when  viewed 


FIG.  3. — From  a  section  of  the  lung  of   a   cat,    stained    with    silver 

nitrate. 

N,    Alveoli    or  air-cells,    lined    with    large,    flat,    nucleated    cells,    with    some 
smaller  polyhedral  nucleated  cells.     (Halliburton  after  Klein  and  Noble  Smith,) 

from  above,  appear  as  flattened  polyhedral  nucleated  plates 
like  a  regular  mosaic.  It  occurs  in  but  a  few  places,  lining 
the  air  sacs  of  the  lungs,  the  mastoid  cells,  the  membranous 
labyrinth  and  crystalline  lens  (Fig.  3). 

(b)  Stratified  squamous  epithelium  is  composed  of  several 
layers  of  epithelial  cells  placed  upon  one  another.  The  deep- 
est layer,  which  rests  upon  the  basement  membrane,  is  com- 
posed of  irregularly  columnar  cells  which  have  their  nuclei 


EPITHELIAL   TISSUES  9 

near  the  lower  border  of  the  cells.  As  they  approach  the 
surface  the  layers  become  flatter  and  more  scale-like  and 
possess  less  vitality.  As  the  outer  layers  are  worn  away  the 
lower  more  vigorous  layers  push  upward  to  the  surface  to 
take  their  place.  In  the  middle  strata,  where  the  cells  are 
polyhedral  in  shape,  we  find  the  layer  of  prickle  cells  which 


FIG.  4. — Vertical  section  of  the  stratified  epithelium  of  the  rabbit's 

cornea. 

a,  anterior  epithelium,  showing  the  different  shapes  of  the  cells  at  various 
depths  from  the  free  surface;  b,  a  portion  of  the  substance  of  cornea.  (Kirkes 
after  Klein.) 

have  minute  projecting  spines  by  which  they  are  connected 
together.  This  layer  is  sometimes  called  the  stratum  spin- 
osum.  Just  below  this  is  the  stratum  germinativum,  or  ger- 
minating layer. 

(2)  Columnar  Epithelium. — (a)  The  simple  columnar  va- 
riety consists  of  a  single  layer  of  column  or  rod-shaped  cells, 
set  upright,  longitudinally  striated  and  containing  oval-shaped 
nuclei.     This  variety  is  found  in  the  lining  of  the  stomach 
and  intestines. 

(b)  In  the  stratified  columnar  variety  the  single  layer  of 
cells  is  replaced  by  several  layers  and  the  superficial  elements 
alone  are  typical.  This  type  is  found  in  the  vas  deferens. 
Ciliated,  it  occurs  in  the  Eustachian  tube,  lachrymal  ducts, 
respiratory  part  of  the  nasal  fossae,  ventricle  of  larynx,  tra- 
chea and  bronchi,  epididymis  and  first  part  of  vas  deferens. 

(3)  Modified    Epithelium. — (a)    Ciliated    epithelium    is 
more  common  with  the  columnar  variety  than  with  any  other. 


10 


THE  ELEMENTARY  TISSUES 


Each  of  the  ciliated  epithelial  cells  presents  on  its  free  sur- 
face twenty  or  more  small,  hair-like,  protoplasmic  append- 
ages, called  cilia.  During  life  these  small  processes  are  in 
constant  rapid  motion,  waving  in  a  direction  toward  the  out- 


FIG.  5. — Ciliated  epithelium  of  the  human  trachea. 

a,  layer  of  longitudinally  arranged  elastic  fibers;  b,  basement  membrane;  c, 
deeoest  cells,  circular  in  form;  d,  intermediate  elongated  cells;  e,  outermost 
layer  of  cells  fully  developed  and  bearing  cilia.  X  350.  (Kirkes  after  Kollikcr.) 

let  of  the  cavity  in  which  th.ey  are  found.  In  the  genital  or- 
gans they  are  important  in  bringing  together  the  male  and 
female  elements  of  reproduction,  while  in  the  respiratory 
tract  they  are  concerned  in  aiding  the 
passage  of  the  mucus  and  in  the  ex- 
pulsion of  foreign  bodies. 

(b)  Goblet  cells  are  found  on  all 
surfaces  covered  by  columnar  epithe- 
lium, but  especially  in  the  large  intes- 
tine.    They  secrete  mucin,  the  main 
constituent  of  mucus,  which  so  dis- 
tends the  cell  that  it  ultimately  bursts 
and  sets  free  its  contents. 

(c)  Pigment ed  epithelium  is  ordi- 
nary epithelium,  the  protoplasm  of  which  has  become  in- 
vaded and  colored  by  foreign  matter,  such  as  fat,  proteid, 
etc.    Such  cells  are  constant  in  the  deeper  layers  of  the  epi- 


FIG.  6. — Goblet  cells. 

(Halliburton  after  Klein.) 


CONNECTIVE  TISSUES  II 

dermis,  especially  of  certain  races,  as  the  negro.     It  is  also 
found  in  the  choroid  coat  of  the  eye. 

(d)  Glandular  epithelium  may  be  columnar,  spherical  or 
polyhedral  in  shape.    It  is  found  lining  the  terminal  recesses 
of  secreting  glands.    The  protoplasm  of  the  cell  usually  con- 
tains the  material  which  the  gland  secretes. 

(e)  N euro-epithelium  is  the  name  given  to  that  covering 
those  parts  toward  which  the  nerves  of  special  sense  are  di- 
rected, and  is  epithelium  of  the  highest  specialization.     It 
occurs  in  the  retina,  the  membranous  labyrinth  and  in  the  ol- 
factory and  taste  cells. 

The  Connective  Tissues. 

All  these  tissues  are  developed  from  the  same  embryonal 
elements,  but  present  varieties  differing  widely  in  appearance 
and  properties.  They  are  characterized  by  the  preponder- 
ance of  the  inter-cellular  over  the  cellular  elements.  The 
physical  characteristics  of  these  tissues  are  very  important 
and  depend  mostly  upon  the  intercellular  elements.  Their 
purpose  in  the  animal  economy  is  to  furnish  a  supporting 
and  connecting  framework  for  the  body.  In  the  embryonal 
state  the  intercellular  substance  is  semi-fluid  and  gelatinous. 
Later,  in  adult  connective  tissue  it  becomes  more  definitely 
formed,  although  it  is  still  soft.  In  adult  areolar  tissue  the 
intercellular  substance  becomes  tough  and  yielding.  When 
this  intercellular  substance  becomes  impregnated  with  cal- 
careous salts  we  hav6  bone.  However,  during  all  these 
changes  in  the  intercellular  substance  little  or  no  change  has 
taken  place  in  the  cellular  structure.  The  bone-corpuscle,  the 
cartilage-cell,  the  tendon-cell  and  the  connective  tissue-cell 
are  all  essentially  identical. 

The  divisions  of  connective  tissue  are:  (i)  Mucous  Tis- 
sue, (2)  Reticular  Tissue,  (3)  Fibrous  Tissue,  (4)  Adipose 
Tissue,  (5)  Cartilage,  (6)  Bone. 


12 


THE  ELEMENTARY  TISSUES 


(i)  Mucous  Tissue.— This  is  the  most  immature  form  of 
connective  tissue  and  consists  of  a  loose  protoplasmic  net- 
work having  a  gelatinous  intercellular  substance.  It 


is 


FIG.  7. — Tissue  of  the  jelly  of  Wharton  from  umbilical  cord. 

a,    Connective-tissue    corpuscles;    b,    fasciculi    of    connective-tissue    fibers;    c, 
spherical  cells.      (Halliburton  after  Frey.) 


FIG.  8. — Reticular  tissue  from  a   lymphatic   gland,    from    a    section 

which  has  been  treated  with  dilute  potash.     {Halliburton  after 

S  chafer.} 

found  in  Wharton's  jelly  in  the  embryo  and  in  certain  tu- 
mors, known  as  myxomata. 

(2)  Reticular  Tissue. — This  is  composed  chiefly  of  a  net- 
work of  connective-tissue  cells  which  enclose  a  mass  of  lym- 


CONNECTIVE  TISSUES  13 

phoid  elements.  It  forms  the  connecting  layer  beneath  the 
skin,  the  submucous  and  subserous  tissues,  and  the  layer  be- 
tween the  muscles.  It  receives  its  name  on  account  of  the 
areolse  or  spaces  within  its  substance,  which  permit  the  adja- 
cent parts  to  move  easily  upon  one  another.  It  consists  of 
white  and  yellow  fibers  in  about  an  equal  proportion. 

(3)  Fibrous  Tissue. — This  variety  includes  all  the  more 
usual  forms  of  connective  tissue  found  in  the  various  parts 
of  the  body.  It  may  be  further  subdivided  into :  (a)  White 
fibrous  tissue,  (b)  yellow  elastic,  and  (3)  loose  fibrous  or 
areolar  tissue. 


FIG.    o.— Bundles    of    the    white    fibers     of    areolar    tissue    partly 
unravelled.      (Kirkes  after  Sharpey.} 

(a)  White  fibrous  tissue  is  composed  of  groups  or  bundles 
of  fibers  which  have  a  wavy  longitudinal  striation.  It  is 
tough  and  inelastic  and  forms  ligaments,  tendons  and  mem- 
branes in  various  parts  of  the  body.  Chemically  this  tissue 
is  composed  of  a  complex  albuminoid  substance,  collagen. 
Upon  being  treated  with  acetic  acid  the  fibers  become  swollen 
and  transparent  and  finally  invisible. 


14  THE  ELEMENTARY  TISSUES 

(b)  Yellow  elastic  tissue  is  composed  of  bundles  of  long, 
regular  and  branched  fibers.  It  is  characterized  by  its 
marked  elasticity.  It  is  found  in  the  vocal  cords,  longitudi- 
nal coat  of  the  trachea  and  bronchi,  inner  coat  of  blood-ves- 
sels, especially  the  large  arteries,  and  in  some  ligaments.  Its 
yellow-tinted  fibers  are  seen  in  parallel  waves  and  are 
larger  than  those  in  the  white  tissue.  They  sometimes  form 
a  web-like  layer,  as  in  the  fenestrated  layer  of  Hienle  in  the 
arteries. 


FIG.      10. — Elastic     fibers  FIG.   11.— Group  of  fat-cells   (F  c) 

from  the  ligamenta  stibflava.  with  capillary  vessels    (c).     (Kirkcs 

X200.       (Halliburton     after  after  Noble  Smith.} 
Sharpey.] 

(4)  Adipose  Tissue. — This  tissue  exists  in  nearly  all  parts 
of  the  body  except  the  subcutaneous  tissue  of  the  eyelids,  the 
penis  and  scrotum,  the  nymphse  and  in  certain  parts  of  the 
lungs.  It  is  nearly  always  found  within  the  meshes  of  are- 
olar  tissue,  where  it  forms  lobules  of  fat.  Fatty  matter  in 
the  form  of  oily  tissue  is  found  in  the  brain,  liver,  blood  and 
chyle.  The  tissue  is  densest  beneath  the  skin,  especially  of 


CONNECTIVE  TISSUES 


the  abdomen,  around  the  kidneys^  between  the  furrows  on 
the  surface  of  the  heart  and  in  bone  marrow.  It  has  a  rich 
blood  supply. 

(5)  Cartilage. — Those  tissues  in  which  the  intercellular 
substance  has  undergone  condensation  until  it  appears  homo- 
gqneous  are  classified  as  cartilage. 
Consequent    upon    the   differences 
exhibited  by  the  intercellular  ma- 
trix it  is  divided  into  the   follow- 
ing   varieties:    (a)    Hyaline,    (b) 
elastic,  and  (c)  fibrous. 

(a)  Hyaline  cartilage  is  of  firm 
consistence,   considerable   elasticity 
and  is  pearly  blue  in  color.     It  is 
enveloped  in  a  fibrous  membrane, 
the  perichondrium,  from  the  vessels 
of  which  it  derives  its  nutrition.   It 
is  composed  of  cells,  irregular  in 
outline  and  arranged  in  patches  of 
various  shapes,  which  are  embedded 
in  a  homogeneous  matrix.       The 
articular    surfaces    of    bones,    the 
costal    cartilages,   and    the    larger 
cartilages  of  the  larynx,  trachea  and 
bronchi,    and    also,    those    of    the 
nose     and     Eustachian     tube     are 
formed  of  this  variety.   In  the  em- 
bryo  this    cartilage    forms    nearly 
the    whole    of    the     future    bony 
skeleton. 

(b)  Elastic  cartilage  is  characterized  by  the  presence  of  an 
abundance  of  elastic  fibers  in  the  matrix.     These  resemble 
those  found  in  the  yellow  variety  of  elastic  tissue.    This  va- 
riety or  cartilage  is  found  in  the  external  ear,  epiglottis, 
cornicula  laryngis  and  Eustachian  tube. 

(c)  Fibrous  cartilage  is  characterized  by  the  presence  of  a 


FIG.  12.  —  Sections  of 
Hyaline  cartilage. 

a,  Fibrous  layer^  of  peri- 
chondrium ;  b,  genetic  layer  of 
perichondrium;  c,  youngest 
chondroblasts ;  d,  older  chon- 
droblasts;  e,  capsule;  f,  cells; 
g,  lacuna.  (Radasch.) 


i6 


THE  ELEMENTARY  TISSUES 


large  amount  of  white  fibrous  tissue  in  the  matrix.  It  com- 
bines the  toughness  and  flexibility  of  fibrous  tissue  with  the 
firmness  and  elasticity  of  cartilage.  It  is  found  chiefly  in  the 
intervertebral  disks,  the  symphyses  and  interarticular  disks 
of  certain  joints,  and  lining,  bony  grooves  for  tendons. 
Chemically,  cartilage  is  complex,  consisting  of  a  mixture 


FIG.  13. — Elastic  fibro-cartilage, 

Showing  cells  in  capsules  and  elastic  fibers  in  matrix.     (From  Yeo  after  Cadiat). 

of  collagen,  chondro-mucoid  and  albuminoid  substances.  On 
boiling,  it  yields  a  substance  known  as  chondrin,  which  on 
cooling  turns  to  gelatin. 


FIG.   14. — 'White  fibro-cartilage.     (Radasch.} 

(6)  Bone. — Bone  is  a  dense  form  of  connective  tissue  con- 
stituting the  skeleton  or  framework  of  the  body.    It  serves  to 


CONNECTIVE  TISSUES  I/ 

protect  vital  organs  in  the  skull  and  trunk  and  acts  as  levers 
which  are  worked  by  the  muscles  in  the  limbs.  The  tissue 
is  characterized  by  the  deposit  of  calcareous  or  lime  salts 
within  its  intercellular  substance,  to  which  its  well-known 
hardness  is  due.  Most  bones  may  be  divided  into  an  outer 
layer  of  compact  bone  and  an  inner  layer  of  spongy  or  can- 
cellated bone. 


FIG.  15. — Transverse  section  of  compact  bony  tissue   (of  humerus). 

Three  of  the  Haversian  canals  are  seen,  with  their  concentric  rings;  also  the 
lacunae,  with  the  canaliculi  extending  from  them  across  the  direction  of  the 
lamellae.  The  Haversian  apertures  were  filled  with  air  and  debris  in  grinding 
down  the  section,  and  therefore  appear  black  in  the  figure,  which  represents 
the  object  as  viewed  with  transmitted  light.  The  Haversian  systems  are  so 
closely  packed  in  this  section  that  scarcely  any  interstitial  lamellae  are  visible. 
X  150.  (Kirkes  after  Sharpey.) 

Microscopically  bone  is  seen  to  consist  of  numbers  of  os- 
seous layers  or  lamellae,  arranged  as,  (a)  circumferential  la- 
mellae which  are  arranged  parallel  to  the  inner  and  outer 
surfaces  of  the  bone,  (b)  Haversian  lamella  which  are  ar- 
ranged concentrically  around  the  Haversian  canals  and  (c) 
interstitial  lamella,  which  are  arranged  irregularly  so  as  to 


1 8  THE  ELEMENTARY  TISSUES 

fill  in  the  spaces  which  the  other  lamellae  do  not  fill.  The 
Haversian  canals  are  minute  longitudinal  channels,  each  sur- 
rounded by  its  lamellae  within  which  run  still  smaller  longi- 
tudinal channels,  called  lacunas.  Connecting  the  main  chan- 
nel and  the  lacunae,  and  radiating  in  all  directions  between 
them  are  other  very  minute  channels  known  as  canaliculi. 
Each  Haversian  canal  with  its  surrounding  lamellae,  lacunae 
and  canaliculi  composes  an  Haversian  system. 

A  fibrous  membrane,  the  periosteum,  forms  the  outer 
covering  of  all  bones  except  when  they  are  covered  with  car- 
tilage. It  consists  of  two  layers,  an  outer  fibrous  and  an  in- 
ner fibre-elastic  layer.  However,  during  the  period  of  devel- 
opment a  third  layer,  the  osteogenetic  layer,  lies  to  the  inte- 
rior. It  possesses  a  rich  blood  supply  which  nourishes  the 
subjacent  bone,  and  contains  cells  which  later  become  bone- 
forming  elements — the  osteoblasts. 

Bone  marrow  is  the  highly  vascular  substance  found 
within  the  central  cavity  of  the  long  bones  and  the  Haversian 
canals.  It  may  be  divided  into  two  classes:  (i)  Red  bone 
marrow,  and  (2)  yellow  marrow.  In  early  childhood  all  the 
marrow  in  the  bones  is  red  or  has  a  reddish  tint,  but  in  adult 
life  we  find  two  kinds — the  red  and  the  yellow. 

( i )  Red  bone  marrow  is  classed  as  one  of  the  blood- 
forming  organs  since  it  plays  an  important  role  in  the  for- 
mation of  the  blood.  When  stained  and  examined  under  the 
microscope  it  is  found  to  consist  of  a  delicate  connective- 
tissue  reticulum  which  supports  the  blood-vessels  and  con- 
tains in  its  meshes  numerous  cells.  On  the  outside  of  the 
marrow,  next  to  the  bone,  we  find  a  thin  fibrous-tissue  coat, 
the  endosteum,  which  lines  the  medullary  cavity  and  extends 
into  the  larger  Haversian  canals.  The  more  numerous  of 
the  cells  found  in  the  red  marrow  are,  (a)  the  myelocytes, 
which  are  very  numerous  and  contain  several  different  va- 
rieties of  granules,  (b)  the  eoslnophiles,  which  are  few  in 
number,  but  which  are  conspicuous  by  the  presence  of  coarse 
granules  within  the  cytoplasm,  which  are  colored  intensely 


MUSCULAR  TISSUES 


by  acid  stains,  such  as  eosin,  (c)  the 
giant  cells,  which  are  very  large,  but 
contain  only  one  nucleus,  (d)  the 
erythroblasts,  which  are  nucleated 
red  blood-cells.  In  addition  to  these 
the  red  marrow  contains  mast-cells, 
fat-cells  and  osteoclasts,  or  multinu- 
clear  giant-cells. 

(2)  Yellow  bone  marrow  is  formed 
from  red  marrow  by  the  infiltration  of 
fat-cells  which  convert  it  into  adipose 
tissue.  When  examined  in  section 
yellow  marrow  resembles  ordinary 
fat-tissue,  consisting  chiefly  of  large 
compressed  spherical  fat-cells  which 
are  supported  by  a  recticulum  of  con- 
nective tissue.  Yellow  marrow  is 
found  in  all  the  adult  long  bones,  ex- 
cept at  their  extremities. 

The  Muscular  Tissues. 

The  chief  characteristic  of  muscu- 
lar tissue  which  distinguishes  it  from 
all  other  tissues  is  its  marked  contrac- 
tility. This  variety  of  tissue  may  be 
divided  into  three  large  groups:  (i) 
Striated  muscle,  (2)  cardiac  muscle, 
and  (3)  smooth  muscle. 

(i)  Striated  or  voluntary  muscle 
makes  up  the  greater  part  of  all  the 
skeletal  muscles  by  means  of  which 
all  voluntary  movements  are  made. 
In  addition  to  this,  it  constitutes  the 
walls  of  the  abdomen,  and  a  few  of 
the  muscles  connected  with  the  mid- 
dle ear,  tongue,  pharynx,  larynx,  dia- 


FIG.    1 6. — Two    fibers 
of   striated   muscle, 

In  which  the  contractile 
substance,  m,  has  been  rup- 
tured and  separated  from 
the  sarcolemma,  a  and  j; 
p,  space  under  sarcolemma. 
(From  Yeo  after  Ranvier.) 


2O  THE  ELEMENTARY  TISSUES 

phragm,  and  generative  organs.  This  group  of  muscular  tis- 
sue is  composed  of  bundles  of  fibers,  each  fiber  of  which  is 
derived  from  a  single  cell  which  has  many  nuclei.  Each 
fiber  is  enclosed  in  a  thin,  homogeneous,  elastic  membrane, 
the  sarcolemma.  The  fibers  are  composed  of  a  semi-fluid 
and  viscous  material  which  is  called  the  muscle  plasma.  The 
muscle  plasma  consists  of  two  elements,  the  fibrils  and  the 
sarcoplasm.  The  fibrils  which  are  long  and  thread-like,  run- 
ning the  entire  length  of  the  .fiber,  consist  of  alternating  light 


FIG.  17. — Striated  muscular  tissue  of  the  heart, 

Showing  the  trelliswork   formed  by  the  short  branching  cells,   with   central 
^  nuclei.     (Yeo.) 

and  dark  segments  which  fall  together  in  the  different  fibrils 
and  give  the  muscle  its  characteristic  striated  appearance. 
The  sarcoplasm,  which  varies  greatly  in  the  striated  muscle 
of  different  animals,  fills  in  the  space  between  the  fibrils. 
From  a  study  of  comparative  physiology  it  is  assumed  that 
the  fibrils  are  the  contractile  element  of  the  muscle  fiber, 
while  the  sarcoplasm  serves  a  general  nutritive  function. 

Striated  muscular  tissue  is  very  richly  supplied  with  .blood- 
vessels.    The  larger  arteries  and  their  accompanying  veins 


MUSCULAR  TISSUES 


21 


enter  the  muscle  along  connective  tis- 
sue septa  and  then  break  up  into 
smaller  branches  and,  finally,  into  a 
capillary  network  which  supplies  the 
individual  muscle  fibers.  They  are 
also  supplied  with  lymphatics  which 
occupy  the  clefts  in  the  connective-tis- 
sue septa  around  the  fibers.  There 
are  also  definite  lymph-vessels  which 
accompany  the  blood-vessels  within 
the  muscle.  This  tissue  is  also  sup- 
plied with  both  motor  and  sensory 
nerves,  by  means  of  which  the  stimuli 
are  carried  to  and  from  the  muscle 
fibers. 

(2)  Heart  muscle  occupies  an  in- 
termediate position  between  the  stri- 
ated voluntary  muscle  and  the  non- 
striated  involuntary  muscle  tissue.    It 
is  characteristic  in  that  it  is  striated 
and  involuntary.     The  following  is  a 
brief    summary    of    its    chief    distin- 
guishing features:   (i)  Its  fibers  are 
united  with  each  other  at  frequent  in- 
tervals   by    short   branches,    (2)    its 
fibers  are  smaller  and  their  striation  is 
less  marked  than  in  voluntary  mus- 
cle,   (3)   it  has  no  sarcolemma,  and 
(4)  its  nuclei  are  situated  within  the 
substance  of  the  fiber  and  not  upon  it. 

(3)  Smooth  or  involuntary  muscle 
occurs    in    bundles    and    thin    sheets 
chiefly  in  viscera  and  blood-vessels. 
Its  general  distribution  may  be  out- 
lined as  follows:   (i)   It  is  found  in 
the  digestive  tract  from  the  middle  of 


FIG.  18.  —  Cells  of 
smooth  muscle  tissue 
from  the  intestinal 
tract  of  rabbit.  (From 
Yeo  after  Ranvier.} 

A  and  B,  muscle-cells  in 
which  differentiation  of  the 
protoplasm  can  be  well  seen. 


22  THE   ELEMENTARY   TISSUES 

esophagus  to  the  anus,  (2)  in  the  capsule  of  the  pelvis  of  the 
kidney,  (3)  in  the  trachea  and  bronchi,  (4)  in  the  ducts  of 
glands,  (5)  in  the  gall-bladder,  (6)  in  the  vas  deferens  and 
seminal  vesicles  of  the  male  reproductive  organs,  (7)  in  the 
uterus,  vagina  and  oviducts  of  the  female  reproductive  or- 
gans, (8)  in  the  blood-vessels  and  lymphatics,  (9)  in  the 
iris,  ciliary  bodies  and  eye-lids,  and  ( 10)  in  the  hair  follicles, 
sweat  glands,  and  skin  of  the  scrotum  and  in  some  other 
places  throughout  the  body. 

The  structural  unit  of  smooth  muscle  is  the  fiber-cell 
which  is  a  delicate  spindle  with  its  nucleus  usually  situated 
nearer  one  end  than  the  other.  The  nuclei  of  the  fiber-cells 
are  usually  elongated  and  oval.  These  fiber-cells  are  held 
together  by  a  delicate  connective-tissue  network  which  is 
composed  of  both  white  and  elastic  fibers.  Smooth  muscle 
is  very  poorly  supplied  with  blood-vessels  in  comparison  to 
striated  muscle.  The  blood-vessels  run  along  the  connective- 
tissue  septa  and  small  branches  are  distributed  to  the  fiber- 
cells.  The  lymphatics,  also,  follow  the  connective-tissue 
septa.  The  nerves  which  supply  the  smooth  muscle  are  from 
the  sympathetic  system. 

The  Nervous  Tissues. 

These  tissues  will  be  considered  under  the  chapter  on  the 
Physiology  of  the  Nervous  System. 


CHAPTER  III. 
PHYSIOLOGICAL  CHARACTERISTICS   OF   MUSCLE. 

WHEN  a  muscle  is  acted  upon  by  a  weight  it  extends 
quite  readily,  but  as  soon  as  the  weight  is  removed  the  mus- 
cle resumes  its  normal  shape.  This  illustrates  the  extensi- 
bility and  elasticity  of  muscular  tissue.  The  muscles  all  over 
the  body  are  in  a  constant  state  of  elastic  tension,  which 
causes  them  to  be  of  greater  value  as -a  support  to  the  body 
skeleton.  A  muscle  which  is  in  a  state  of  elastic  tension  con- 
tracts more  readily  and  forcibly  than  one  which  is  relaxed. 

Under  ordinary  conditions  a  muscle  receives  the  stimulus 
which  causes  it  to  contract  through  its  motor  nerve  from  the 
central  nervous  system.  If  this  nerve  be  cut,  the  muscle  is 
paralyzed.  However,  it  has  been  demonstrated  that  a  mus- 
cle which  has  its  nerve  cut  may  still  be  made  to  contract  by 
applying  an  artificial  stimulus,  as  an  electrical  shock.  But 
such  a  muscle  would  still  have  its  nerve  endings  in  the  mus- 
cle undestroyed,  and  hence,  this  would  not  prove  that  the 
muscle  has  independent  contractility.  Still,  if  the  nerve  is 
severed  and  the  nerve  endings  are  destroyed,  e.  g.,  by  a  drug, 
we  find  that  the  muscle  will  still  respond  to  an  electrical  stim- 
ulus. This  shows  that  muscular  tissue  has  independent  irri- 
tability. Hence,  striated  muscular  tissue  possesses  indepen- 
dent contractility,  by  which  is  meant  that  its  power  of  short- 
ening is  due  to  active  processes  developed  in  its  own  tissue, 
and  independent  irritability,  by  which  is  meant  that  it  may 
enter  into  contraction  by  artificial  stimuli  applied  directly  to 
its  own  substance. 

If  we  isolate  a  muscle  and  stimulate  it,  we  get  a  simple 
contraction.  If  the  end  of  this  muscle  is  attached  to  a  lever 

23 


24  PHYSIOLOGICAL   CHARACTERISTICS   OF   MUSCLE 

connected  with  a  revolving  drum,  we  get  a  simple  muscle 
curve  (Fig.  19).  The  time  required  for  a  simple  contraction 
varies  with  the  muscles  of  different  animals,  and  also  with 
different  muscles  of  the  same  animal.  After  the  muscle  is 
stimulated  (Fig.  19),  an  appreciable  time  elapses,  the  latent 
period,  before  it  contracts,  which  is  about  Moo  second.  Then 
the  muscle  passes  into  the  stage  of  contraction,  during  which 
time  the  lever  rises.  Immediately  it  relaxes  and  elongates 
and  the  lever  again  descends  to  the  base  line.  The  whole 
contraction  occupies  about  Mo  second. 


FIG.   19. — Simple   muscle  curve.      (Halliburton.) 

Those  factors  which  modify  the  character  of  a  simple 
muscle  curve  are,  (a)  the  strength  of  the  stimulus,  (b)  the 
amount  of  the  load,  (c)  the  influence  of  fatigue,  (d)  the  ef- 
fect of  temperature,  and  (e)  the  effect  of  veratrine. 

(a)  A  stimulus  which  is  just  strong  enough  to  produce  a 
contraction  is  called  a  minimal  stimulus.  As  the  strength  of 
the  stimulus  is  increased  the  amount  of  the  contraction, 
which  is  represented  by  the  height  of  the  curve,  is  increased. 
This  continues  until  a  certain  point  is  reached,  the  maximal 


PHYSIOLOGICAL   CHARACTERISTICS   OF    MUSCLE  25 

stimulus,  then  an  increase  in  the  stimulus  produces  no  in- 
crease in  the  contraction. 

(b)  As  the  weight  of  the  load  is  increased  the  contrac- 
tion becomes  less  until  a  weight  is  reached  which  the  muscle 
is  unable  to  raise.     Also,  the  latent  period  is  longer  with  a 
heavy  than  with  a  light  load. 

(c)  If  we  apply  a  series  of  successive  stimuli  to  a  muscle 
we  notice  that  at  first  the  contractions  improve  with  each 
successive  stimulus  which  is  due  to  the  beneficial  effect  of 
contraction.    Later  the  contractions  get  less  and  less.    As  the 
contractions   get   less,   the   period   of    contraction   becomes 
longer,  the  latent  period  is  increased  and  the  period  of  re- 
laxation becomes  very  much  longer.    As  the  period  of  relax- 
ation becomes  longer,  the  muscle  fails  to  return  to  its  nor- 
mal length  before  a  second  stimulus  arrives,  so  that  the  orig- 
inal base  line  is  not  reached  at  all.    This  condition  is  known 
as  contracture. 

(d)  By  varying  the  temperature  of  a  muscle  we  find  that  it 
causes  a  variation  in  the  extent  and  duration  of  its  contrac- 
tions.   Thus,  by  beginning  at  o°  C..  and  increasing  the  tem- 
perature, we  find  that  the  contractions  increase  up  to  5°-9° 
C.  and  then  decrease  up  to  i5°-i8°  C.     After  this  point  is 
reached   they   again   increase   reaching   their   maximum   at 
26°-3O°  C.     This  maximum  is  much  greater  than  the  first 
maximum  which  was  reached  at  5°-9°  C.    As  the  tempera- 
ture is  still  increased,  the  contractions  decrease  rapidly  until 
at  about  37°  C.  irritability  is  entirely  lost.     If  the  tempera- 
ture is  increased  to  about  42°  C.  heat  rigor  makes  its  appear- 
ance due  to  the  coagulation  of  the  muscle  plasma. 

(e)  Veratrine  is  an  alkaloid  which  exerts  a  peculiar  effect 
upon  the  contraction  of  muscle.    By  injecting  it  into  an  ani- 
mal before  the  muscle  is  removed  the  following  effects  are 
noted:  (i)  The  phase  of  shortening  is  not  altered,  but  the 
period  of  relaxation  is  very  much  prolonged,  and  (2)  there 
is  a  secondary  rise  in  the  curve  of  relaxation. 


26 


PHYSIOLOGICAL   CHARACTERISTICS  OF    MUSCLE 


Effect  of  Two  or  More  Successive  Stimuli.— If  a  muscle 
receives  two  successive  stimuli  a  sufficient  length  of  time 
apart,  two  curves  of  contraction  are  produced,  the  second 
being  a  little  higher  than  the  first  (beneficial  effect  of  con- 
traction). However,  if  the  second  stimulus  arrives  before 
the  period  of  relaxation  is  complete,  a  secondary  rise  is  pro- 
duced which  is  called  superposition  or  summation  of  effects. 


n 


FIG.  20. 

I,  Two  successive  submaximal  contractions.  //,  A  series  of  contractions  in- 
duced by  12  induction-shocks  in  a  second.  ///,  Marked  tetanus  induced  by 
rapid  shocks.  (Landrois.) 

If  the  two  stimuli  occur  close  enough  together,  the  result  will 
be  one  curve  which  is  greater  than  either  would  have  pro- 
duced separately.  This  is  called  summation  of  stimuli.  If, 
instead  of  just  two  stimuli,  a  number  of  stimuli  are  applied 
very  close  together,  we  get  the  effect  shown  in  (II).  If 
these  stimuli  occur  still  closer  together  the  effect  shown  in 
(III)  is  produced  which  is  called  tetanus.  When  the  stim- 
uli occur  so  as  to  allow  partial  relaxation  between  each 
stimulus,  (II)  the  effect  is  called  incomplete  tetanus,  but 
when  no  relaxation  occurs  as  in  (III)  the  effect  is  complete 
tetanus. 


CHAPTER  IV. 
SECRETION. 

Secretion  and  Excretion. — Ordinarily  the  product  of 
glandular  activity  is  spoken  of  as  a  secretion.  On  the  one 
hand,  glands  may  take  from  the  blood  substances  which  are 
formed  in  that  fluid,  which  would  accumulate  and  pro- 
duce detrimental  effects  if  not  removed,  and  which  are  dis- 
charged from  the  body.  On  the  other  hand,  glands  may 
form  out  of  materials  furnished  by  the  blood  substances 
which  are  peculiar  to  that  gland's  activity,  which  have  an 
office  to  perform  in  the  economy,  which  do  not  accumulate 
on  removal  of  the  gland,  and  which  are  not  discharged  from 
the  body.  The  product  in  the  first  case  is  an  excretion,  in 
the  second  case  a  secretion.  But  when  it  comes  to  naming 
an  exclusively  excretory  or  exclusively  secretory  gland, 
the  task  is  found  to  be  practically  impossible.  Probably  the 
most  typical  excretion  of  the  body  is  the  urine,  yet  there'  are 
in  the  urine  substances,  like  hippuric  acid,  etc.,  which  are 
undoubtedly  formed  by  the  kidney,  and  which  do  not  pre- 
exist in  the  blood.  The  succus  entericus,  e.  g.,  would  seem 
as  typical  a  secretion  as  it  is  possible  to  find,  but  not  infre- 
quently it  contains  urea  when  the  activity  of  the  kidney  is 
impaired,  to  say  nothing,  under  normal  conditions,  of  the 
water  and  salts  which  are  taken  as  such  from  the  blood.  The 
liver  is  notable  in  its  secreto-excrementitious  action.  While 
the  desirability  of  thus  separating  the  glands  into  secretory 
and  excretory  and  their  products  into  secretions  and  excre- 
tions is  granted,  the  impossibility  of  such  a  division  is  appar- 
ent. 

It  is  possible  in  most  cases  to  apply  the  distinction  to  the 
separate  constituents  of  the  product  of  a  particular  gland, 

27 


28  SECRETION 

but  not  to  the  product  as  a  whole.  In  view  of  these  facts,  at- 
tention will  be  given  in  this  chapter  to  several  glands  which 
manifestly  produce  excretions  as  well  as  secretions.  The 
action  of  the  kidney  and  sweat  glands  is  so  predominantly 
excretory  that  they  are  treated  separately.  In  what  follows 
the  term  "secretion"  cannot  always  be  taken  as  meaning  a 
true  secretion,  for  it  is  customary  and  convenient  to  speak 
of  the  "secretion  of  urine,"  for  example. 

Glands. — If  we  conceive  of  a  single  layer  of  secreting  epi- 
thelial cells  supported  by  a  thin  basement  membrane,  and 
then  this  structure  invaginated  or  folded  in  upon  itself,  so 
that  the  two  layers  of  epithelium  face  each  other  with  a 
greater  or  less  interval  between  them,  with  the  basement 
membrane  constituting  the  external  support  for  both,  we 
will  have  in  mind  the  essential  structure  of  a  gland  proper. 
The  invaginated  cells  are  the  gland  cells,  and  the  interval 
between  the  two  layers  of  cells  is  the  lumen.  Whether  the 
invaginated  structure  sends  off  from  itself  secondary  or  ter- 
tiary folds  similar  to  the  original,  or  whether  the  lumen  of 
any  of  these  folds  is  in  the  shape  of  a  simple  tube  or  sac,  or 
both,  is  immaterial.  They  may  all  be  considered  as  identical 
in  nature  with  the  original  invagination  and  only  modifica- 
tions of  its  architecture. 

However,  these  modifications  are  more  or  less  distin- 
guished by  names.  Those  which  become  complex  by  numer- 
ous branchings  of  the  involuted  tube  are  usually  termed 
compound,  as  opposed  to  a  single  simple  fold ;  glands  are 
further  classified,  as  tubular,  racemose,  or  tubulo-racemose, 
according  as  the  termination  of  the  lumen  has  the  shape  of  a 
tube,  or  sac,  or  both.  Thus  a  simple  or  a  compound  gland 
may  belong  to  any  one  of  the  three  last-named  varieties. 
The  crypts  of  Lieberkuhn  are  simple  tubular  glands.  The 
glands  of  Brunner  are  usually  described  as  compound  tubulo- 
racemose  structures. 

In  a  compound  gland  that  portion  which  communicates 
with  the  surface  is  called  the  duct  and  is  supposed  not  to  be 


GLAND  SECRETION  29 

concerned  in  actual  secretion,  but  simply  in  carrying  the  pro- 
duct away  from  the  secreting  terminal  ramifications  of  the 
subdivisions  of  the  involution — which  terminations  are  called 
acini  or  alveoli.  It  follows,  of  course,  that  a  collection  of 
acini  may  discharge  their  secretion  into  the  main  duct  by  a 
smaller  duct — that  is,  that  the  gland  may  have  various  subdi- 
visions of  the  duct  proper. 

Furthermore,  secretions  are  classified  as  external  when 
they  are  discharged  upon  a  surface  communicating  with  the 
external  air,  such  as  the  alimentary  canal,  or  skin,  and 
internal  when  they  are  discharged  upon  surfaces  not  in 
communication  with  the  exterior,  such  as  blood-vessels. 
Both  external  and  internal  secretions  are  liquid  or  semi- 
liquid  in  character,  for  they  must  contain  water  as  a  vehicle 
for  the  salts  and  organic  substances  which  are  present  in  all 
of  them  and  which,  in  fact,  distinguish  them  from  one  an- 
other. 

Glands  in  general  have  been  divided  into  serous  and 
mucous  by  Heidenhain,  according  as  the  secreted  fluid  is 
watery  and  thin,  or  viscid  and  stringy  from  the  presence  of 
mucin.  This  division  is  further  warranted  by  histologic 
differences  in  the  cells  concerned  in  each  kind  of  secretion. 
The  cells  in  a  serous  gland  are  small  and  finely  granular,  and 
are  in  close  apposition  to  each  other.  Those  of  mucous 
glands  are  larger,  almost  square  and  are  definitely  separated. 
Many  glands  contain  both  kinds  of  cells,  but  since  their  se- 
cretion contains  mucin,  such  glands  are  usually  spoken  of  as 
belonging  to  the  mucous  variety.  It  will  be  seen  that  the 
salivary  glands  illustrate  these  varieties. 

Gland  Secretion. — Underneath  the  basement  membrane  of 
a  gland  (that  is,  on  the  side  opposite  the  epithelial  cells) 
ramifies  an  abundant  network  of  blood  and  lymph  capillaries. 
This  anatomical  arrangement  favors  osmotic  transudation 
from  the  vessels,  especially  since  the  pressure  in  the  vessels 
is  normally  greater  than  in  the  acini  and  ducts  of  the  gland. 
Numerous  experiments,  however,  prove  the  inadequacy  of 


30  SECRETION 

simple  osmosis  to  explain  all  the  processes  of  glandular  se- 
cretion, especially  those  connected  with  the  presence  of  or- 
ganic constituents;  while  the  undoubted  presence  of  secre- 
tory nerves  (besides  the  vaso-motor  nerves  to  the  vessels) 
would  seem  to  give  a  priori  evidence  that  the  glandular  epi- 
thelium takes  some  active  part  in  the  formation  of  the  se- 
cretion. Such  an  office  is  granted  to  these  cells,  but  whether 
it  is  of  chemical,  or  a  physical,  or  a  "vital"  character  is  not 
evident. 

The  physiology  of  the  salivary  glands,  the  gastric  and  in- 
testinal glands,  the  pancreas  and  liver  is  taken  up  under  the 
chapter  on  Digestion  in  which  they  are  vitally  concerned. 

Sebaceous  Glands. 

The  sebaceous  glands  (see  Hair-follicles)  are  chiefly  asso- 
ciated with  hair-follicles  and,  existing  wherever  hair  is  to  be 
found,  cover  well-nigh  the  whole  cutaneous  surface.  They 
are  of  the  simple  or  compound  tubular  type,  and  discharge 
their  secretion  into  the  hair- follicle  near  its  outer  extremity. 
The  alveoli  are  lined  by  several  layers  of  cuboidal  epithelial 
cells.  The  cells  of  the  layer  nearest  the  lumen  contain  fatty 
matter,  and  are  thought  to  form  the  secretion  by  breaking 
down  and  being  thrown  off  themselves.  Their  place  is  taken 
by  cells  from  the  deeper  layers,  which  undergo  similar 
changes  and  disintegrate. 

Composition  and  Properties  of  Sebum. — Chemically  se- 
bum is  largely  made  up  of  fatty  matters.  It  also  contains 
cholesterin,  which  is  in  combination  with  a  fatty  acid.  It 
forms  a  thin  coating  over  the  cutaneous  surface,  accounting 
for  the  normal  oiliness  of  the  skin.  It  also  contributes  to  the 
characteristic  softness  of  the  hairs,  and  prevents  their  break- 
ing off  from  brittleness.  Its  presence  over  the  body  surface 
may  have  some  influence  in  regulating  the  loss  of  heat  by 
evaporation. 

Cerumen,  smegma  and  the  secretion  from  the  Nabothian 


MAMMARY  GLANDS  31 

glands  are  only  modified  forms  of  sebum,  and  the  structures 
producing  these  secretions  belong  to  the  class  of  sebaceous 
glands. 

Mammary  Glands. 

Structure. — The  mammary  glands  are  two  in  number  in 
the  human  being,  and  are  loosely  attached  to  the  great  pec- 
toral muscles.  They  are  rudimentary  in  both  sexes  until 
puberty,  and  in  men  throughout  life.  At  puberty  the  gland 
in  the  female  enlarges  markedly,  but  is  never  fully  developed 
before  pregnancy.  At  this  time  the  gland  vesicles  make  their 
appearance,  and  the  rudimentary  ducts  come  to  be  more  and 
more  ramified.  These  ramifications  do  not  reach  their  full 
development,  however,  until  lactation  begins.  The  skin  cov- 
ering the  areola  of  the  nipple  is  dark,  especially  during  preg- 
nancy, and  much  thinner  than  over  other  parts.  The  dark 
color  is  due  to  a  deposit  of  pigment. 

The  mammary  gland  belongs  to  the  compound  tubulo-race- 
mose  type,  and  consists  of  fifteen  or  twenty  lobes  bound 
together  by  areolar  connective  tissue.  Each  lobe  is  made  up 
of  a  number  of  lobules,  containing  the  alveoli  or  secreting 
portions.  The  secretion  from  all  the  alveoli  and  lobules  of  a 
lobe  converges  to  a  single  duct,  which  discharges  its  contents 
upon  the  surface  of  the  nipple  without  anastomosis  with  any 
duct.  There  are,  therefore,  some  fifteen  or  twenty  ducts 
thus  opening  upon  the  surface.  Each  of  them  has  a  dilata- 
tion beneath  the  nipple,  and  it  is  in  these  sinuses  largely  that 
the  milk  accumulates  during  lactation.  When  lactation  has 
ceased  the  ducts  retract,  the  sinuses  disappear,  the  alveoli 
undergo  retrograde  changes,  and  the  whole  gland  is  inclined 
to  become  flabby  and  pendulous.  It  does  not  regain  after 
pregnancy  the  firmness  which  characterized  it  before. 

Secretion  of  Milk. — After  parturition  the  first  discharge 
from  the  gland  is  colostrum,  a  liquid  resembling  milk  in  some 
respects.  In  two  or  three  days  the  true  milk  appears.  Be- 
sides water  and  salts,  all  the  constituents  of  milk  are  formed 


32  SECRETION 

by  the  cells  of  the  mammary  gland.  During  the  period  of 
gestation  the  cells  lining  the  alveoli  are  flat  and  have  only  a 
single  nucleus.  When  they  begin  -to  secrete  they  increase  in 
height,  the  nuclei  divide  and  that  portion  of  the  cell  toward 
the  lumen  undergoes  fatty  degeneration.  This  fatty  ma- 
terial is  extruded  into  the  lumen  and  apparently  constitutes 
a  part  of  the  secretion.  The  liquid  constituents  taken  out 
of  the  blood  probably  hold  the  proteid  and  carbohydrate 
portions  in  solution,  while  the  fatty  particles  constitute  the 
fat  of  the  milk.  Thus  secreted,  the  liquid  accumulates  in  the 
ducts  and  sinuses  until  removed  by  the  infant  or  otherwise. 
The  fact  that  the  secretion  of  milk  in  woman  is  influenced 
by  emotions  of  fear,  grief,  etc.,  is  strong  evidence  of  a  ner- 
vous control  of  the  procedure,  but  proof  of  secretory  fibers 
to  the  cells  has  not  been  established. 

The  quantity  of  food  required  by  the  mother  during  the 
time  the  child  is  nursed  is  increased,  but  no  particular  kind 
of  food  seems  to  be  especially  required.  The  larger  demand 
for  liquids  is  marked,  however,  and  when  the  quantity  of 
milk  is  increased  by  a  large  ingestion  of  liquids,  the  solids  in 
the  secretion  are  not  relatively  diminished. 

Composition  and  Properties  of  Milk. — Human  milk  has 
specific  gravity  of  about  1030,  and  is  not  so  white  or  so 
opaque  as  cow's  milk.  Besides  water,  its  chief  constituents 
are  fats,  lecithin,  cholesterin,  casein  and  lactose,  of  which 
the  two  last  named  are  the  most  important.  Casein  is  the 
main  proteid  constituent.  Lactose  is  very  abundant,  and  is 
responsible  for  the  sweet  taste  and  for  a  large  part  of  the 
nutritive  value  of  the  fluid. 

Thyroid  Gland. 

The  thyroid  gland  consists  of  two  glandular  masses  united 
by  an  isthmus  .of  the  same  structure.  It  lies  in  front  of  the 
trachea  at  the  lower  end  of  the  larynx.  It  consists  of  a  large 
number  of  vesicles  bound  together  by  connective  tissue. 


THYROID  AND  ADRENAL   GLANDS  33 

Each  vesicle  is  lined  by  cuboidal  epithelial  cells,  which  secrete 
a  semi-gelatinous  substance,  colloid. 

It  has  long  been  known  that  the  removal  of  the  whole  thy- 
roid gland,  including  the  parathyroid,  occasioned  marked  in- 
terference with  nutrition  and  other  changes,  the  chief  of 
which  are  disturbances  of  muscular  coordination,  possibly 
convulsions,  emaciation,  apathy,  and  subsequent  death.  There 
is  no  duct  connected  with  the  gland,  and  the  secretion  is 
therefore  an  internal  one.  Very  little  is  known  of  it  except 
that  it  is  necessary  to  the  maintenance  of  life.  If  a  very  little 
of  the  gland  be  left,  or  if,  after  its  complete  removal,  a  small 
bit  of  it  be  transplanted  in  some  other  part  of  the  body,  or  if 
the  animal  be  fed  on  the  thyroid  extract  or  the  fresh  gland, 
the  characteristic  symptoms  do  not  ensue. 

The  muscular  disturbances  direct  the  attention  to  the  cen- 
tral nervous  system  when  an  attempt  is  made  to  explain  the 
occurrences  and  it  is  not  improbable  that  the  effect  of  the 
thyroid  secretion  is  in  some  way  exerted  upon  or  through 
the  central  system.  It  seems  generally  agreed  that  the  thy- 
roid does  discharge  a  secretion  into  the  blood  and  that  it  is 
the  withdrawal  of  some  part  of  that  secretion  from  the  cir- 
culation which  is  responsible  for  the  remarkable  train  of 
symptoms  sequent  upon  its  removal.  This  essential  constit- 
uent is  regarded  by  some  as  being  an  agent  which  destroys 
certain  toxic  principles  in  the  blood,  by  others  as  being  requi- 
site to  the  metabolic  functions  in  the  body  without  destroy- 
ing anything.  .Baumann  has  isolated  from  the  gland  sub- 
stance a  material  containing  a  large  proportion  of  iodine,  to 
which  he  gives  the  name  iodothyrin,  and  it  is  very  probable 
that  this  is  one,  at  least,  of  the  beneficial  substances  in  the 
thyroid  secretion. 

Adrenal  Glands. 

The  adrenal  gland  or  suprarenal  capsules,  resting  upon 
the  upper  ends  of  the  kidneys,  are  ductless  glands  whose  re- 


34  SECRETION 

moval  is  followed  by  weakness,  impaired  nutrition  and  dis- 
turbances in  the  circulation.  Death  usually  supervenes  in 
two  to  four  days.  These  bodies  must  produce  an  internal 
secretion  which  is  removed  by  way  of  ,the  adrenal  veins.  It 
may  destroy  toxic  substances  in  the  blood.  A  solution  in- 
jected into  the  circulation  certainly  affects  the  middle  wall 
of  the  vessels,  causing  contraction,  and  a  heightened  pres- 
sure. The  heart  is  also  notably  inhibited.  It  is  not  thought 
that  the  effect  on  the  vessels  is  brought  about  through  the 
vaso-motor  nerves,  but  by  direct  excitation  of  the  muscular 
substance.  Little  in  fact  is  known  about  the  secretion,  ex- 
cept that  it  is  necessary  to  life.  Abel  has  isolated  an  alkaloid, 
epinephrine,  which  is  claimed  to  be  the  active  principle. 
These  glands  are  the  seat  of  lesions  in  Addison's  disease,  and 
many  cases  of  this  malady  are  at  least  favorably  influenced 
by  the  use  of  adrenal  extract. 

Pituitary  Body. 

The  pituitary  body  lying  in  the  sella  turcica  on  the  superior 
surface  of  the  sphenoid  bone,  also  produces  an  internal  se- 
cretion of  physiological  value.  Its  removal  is  regarded  as 
causing  death.  Hbwell  has  shown  that  injection  of  extract 
from  the  posterior  division  occasions  a  rise  of  temperature 
and  slowing  of  the  heart.  Its  situation  makes  satisfactory 
experiments  very  difficult. 

Testis  and  Ovary. 

The  testes  and  ovaries,  though  not  probably  true  glands, 
also  may  produce  an  internal  secretion  of  obscure  physio- 
logical value.  It  is  not  essential  to  life.  Injections  of  ex- 
tracts from  these  bodies  are  claimed  to  have  a  remarkable 
stimulating  effect  upon  the  nervous  and  muscular  systems. 
In  mental  and  physical  disturbances  occasionally  following 
removal  of  the  ovaries,  gynecologists  often  find  administra- 
tion of  the  ovarian  extract  to  be  beneficial. 


CHAPTER  V. 
THE  BLOOD. 

General  Characteristics. — The  blood  is  a  red,  opaque  and 
viscid  fluid  having  a  characteristic  stale  odor  and  a  salty 
taste.  The  blood  is  heavier  than  water,  having  a  specific 
gravity  in  the  adult  male  of  1.041  to  1.067,  tne  average  being 
about  1.055. 

The  reaction  of  the  blood  is  neutral.  The  nature  of  the 
diet,  either  meat  or  vegetable,  causes  this  neutrality  to  turn 
to  either  an  acid  or  an  alkaline  reaction. 

The  blood  temperature  is  that  of  the  body.  In  the  periphery 
it  is  about  99°  F. ;  in  deeper  vessels  it  varies  from  100°  F. 
to  107°  F. ;  and  in  the  hepatic  veins  it  is  about  107°  F. 

The  Function  of  the  Blood. — The  most  important  physio- 
logical functions  of  the  blood  are:  (i)  It  carries  to  the  tis- 
sues food-stuffs  after  they  have  been  digested,  (2)  it  trans- 
ports to  the  tissues  oxygen  which  it  has  absorbed  from  the 
air  in  the  lungs,  (3)  it  carries  off  from  the  tissues  the  waste 
products  of  metabolism,  (4)  it  transmits  the  internal  secre- 
tions of  glands  to  the  different  parts  of  the  body,  and  (5)  it 
aids  in  equalizing  -the  body  temperature. 

Quantity  and  Distribution  of  the  Blood. — The  quantity  of 
the  blood  in  the  body  is  estimated  at  about  7.5  per  cent,  of 
the  body  weight.  A  man  weighing  150  pounds  has  a  fraction 
over  eleven  pounds  of  blood,  which  is  about  one-thirteenth 
of  the  body  weight. 

The  distribution  is  generally  given  as,  one-fourth  in  the 
heart,  large  arteries,  lungs,  and  veins ;  one-fourth  in  the  liver ; 
one- fourth  in  the  muscles  attached  to  the  skeleton ;  and  the 
other  one- fourth  variously  distributed  to  the  other  organs  of 
the  body. 

35 


36  THE  BLOOD 

Composition  of  Blood. 

The  blood  is  composed  of  a  fluid  part,  the  plasma,  in  which 
float  a  great  mass  of  small  bodies,  the  blood  corpuscles.  The 
plasma  may  be  defined  as  the  blood  minus  the  corpuscles. 
These  are  of  three  varieties :  ( I )  The  red  corpuscles,  or  ery- 
throcytes,  (2)  the  white  corpuscles,  or  leukocytes,  and  (3) 
the  blood  platelets,  or  thrombocytes.  The  plasma  is  a  thin 
slightly  yellowish  fluid  with  a  specific  gravity  of  1.026  to 
1.029.  Hence,  'the  bright  red  color  of  the  blood  is  due  to  the 
red  corpuscles  which  are  held  in  suspension  in  the  plasma. 
The  proportion  of  plasma  to  corpuscles  is  about  two  to  one 
(Ho  well). 

Plasma. 

Chemically,  plasma  is  composed  of  water  and  about  10 
per  cent,  of  solids,  together  with  oxygen,  carbon  dioxide  and 
nitrogen.  A  thousand  parts  of  plasma  contain:  (Hallibur- 
ton.) 

Water     , 902.90 

Solids 97-io 

Proteins:  I.  Yield   of  fibrin    4.05 

2.  Other  proteins 78.84 

Extractives    (including   fat)    5.66 

Inorganic    salts    8.55 

The  most  important  solids  are  the  proteins,  the  chief  of 
which  are:  (i)  Fibrinogen,  (2)  serum  globulin,  and  (3) 
serum  albumin.  Fibrinogen  belongs  to  the  globulin  class  of 
proteins,  but  differs  from  serum  globulin  and  may  be  separ- 
ated from  it.  Fibrinogen  is  the  least  abundant  of  the  pro- 
terns.  Serum  globulin  and  serum  albumin  form  the  chief 
proteins  of  the  plasma.  They  may  be  separated  by  the  use 
of  neutral  salts. 

The  extractives  are  substances  other  than  proteins  which 
may  be  extracted  from  the  dried  residue  by  the  use  of  water, 
alcohol,  or  ether.  The  principal  extractives  are  fats,  sugar, 
lecithin,  cholesterin,  lactic  acid  and  urea. 


RED  BLOOD  CORPUSCLES  37 

The  most  abundant  salt  of  the  plasma  is  sodium  chloride. 
It  forms  from  60  to  90  per  cent,  of  the  total  mineral  matter 
of  plasma.  Potassium  chloride  is  present  in  much  smaller 
amount.  Other  salts  are  the  carbonates,  sulphates  and  phos- 
phates. 

Corpuscles. 

Suspended  in  the  plasma  of  the  blood  we  have  a  cellular 
formed  element  moving  and  functionating.  This  element  is 
the  corpuscular  element  and  is  composed  of  (a)  the  red 
blood  corpuscles,  (b)  the  white  blood  corpuscles,  (c)  the 
blood  platelets. 

(a)  Red  Blood  Corpuscles  or  Erythrocytes. 

General  Description. — The  red  blood  corpuscles  are  circu- 
lar, bi-concave  discs  with  rounded  edges.  They  are  from  7 
to  8  micra  in  diameter  and  2  micra  in  thickness,  so  can  only 
be  seen  with  the  aid  of  the  microscope.  When  looked  at 
singly  they  appear  to  have  a  yellowish-green  color,  collec- 
tively they  are  red. 

Number. — In  males  there  are  about  5,000,000  red  cells  per 
cubic  millimeter ;  in  females  about  4,500,000.  The  propor- 
tion of  reds  to  whites  is  one  white  to  every  500  red. 

Origin  and  Destruction. — The  red  corpuscles  are  continu- 
ally being  destroyed  in  the  body.  It  appears  that  this  de- 
struction occurs  principally  in  the  liver.  As  the  red  cells 
are  thus  destroyed  it  is  natural  to  look  for  a  place  of  manu- 
facture. In  the  embryo  we  find  that  this  generation  takes 
place  in  the  liver  and  in  the  spleen ;  in  the  adult  it  seems  that 
the  manufacture  takes  place  only  in  the  red  marrow  of  the 
bones. 

The  red  corpuscles  are  formed  from  colored,  nucleated 
cells  called  hemoblasts. 

Constituents  of  Red  Blood  Corpuscles. — The  red  blood 
corpuscles  are  made  up  of  65  per  cent,  water  and  35  per 


3o  THE  BLOOD 

cent,  solids.  The  principal  solid  constituents  are  (a)  hemo- 
globin (oxyhemoglobin)  87-95  Per  cent.,  (b)  stroma,  com- 
posed of  fat,  lecithin,  and  cholesterin,  and  (c)  salts,  princi- 
pally potassium  chloride,  and  potassium  phosphate. 

Hemoglobin. — Hemoglobin  is  the  coloring  matter  of  the 
red  cells,  and  is  composed  of  (i)  hematin,  a  pigment  con- 
taining iron,  and  (2)  globin,  a  proteid.  Hemoglobin  is  of 
great  physiological  importance  because  of  its  ability  to  unite 
with  oxygen  and  thus  form  oxyhemoglobin.  By  it  the  blood 


A,  human  colored  blood  corpuscles — i,  on  the  flat;  2,  on  edge;  3,  rouleau  of 
colored  corpuscles.  B,  amphibian  colored  blood  corpuscles — i,  on  the  flat;  2,  on 
edge.  C,  ideal  transverse  section  of  a  human  colored  blood  corpuscle  magnified 
5,000  times  linear — a,  b,  diameter;  c,  d,  thickness.  (Landois.) 

carries  its  oxygen  from  the  lungs  to  the  tissues.  It  also 
unites  to  some  extent  with  carbon  dioxide  and  it  is  thus  that 
carbon  dioxide  is  brought  from  the  tissues.  We  find  oxy- 
hemoglobin chiefly  in  the  arterial  blood,  while  in  venous 
blood  we  find  both  hemoglobin  and  oxyhemoglobin.  In  as- 
phyxiated blood  we  find  only  hemoglobin. 

The  stroma  is  the  colorless  framework  of  the  corpuscles 


BLOOD  PLATELETS  39 

after  the  coloring  matter  is  dissolved  out.    The  hemoglobin 
is  ensnared  in  the  stroma. 


(b)  White  Blood  Corpuscles  or  Leukocytes. 

General  Description. — The  white  blood  corpuscles  or  leu- 
kocytes are  large,  colorless,  nucleated  cells  with  no  general 
form,  but  which  are  capable  of  changing  their  form  by  ame- 
boid movement. 

Number. — The  number  of  leukocytes  varies  from  seven  to 
ten  thousand  per  cubic  millimeter. 

Function. — The  white  corpuscles  are  not  under  the  control 
of  the  central  nervous  system,  but  are  controlled  by  some 
chemotaxic  force.  They  are  able  to  go  and  come  by  ame- 
boid movement  through  the  stromata  of  capillary  walls  and 
wander  here  and  there  in  the  tissues.  It  is  this  that  gives 
them  their  name  of  wandering  cells. 

White  blood  corpuscles  are  of  importance  from  a  physio- 
logical standpoint,  because  of  this  ability  to  wander.  They 
can  transfer  undissolved  substances  from  one  part  of  the 
body  to  another  and  can  destroy  and  remove  foreign  sub- 
stances and  harmful  microorganisms. 

The  power  they  have  of  ingesting  foreign  substances  is 
called  phagocytosis.  They  will  migrate  in  large  numbers 
and  surround  a  foreign  object  and  endeavor  to  remove  it 
from  the  tissue.  They  have  the  power  of  liquefying  tissue 
and  it  is  this  liquefied  tissue  mixed  with  the  dead  bodies  of 
white  corpuscles  that  is  known  as  pus. 

(c)  Blood  Platelets. 

These  are  colorless  discs  about  one-third  to  one-fourth  the 
size  of  red  blood  corpuscles.  Some  claim  for  them  the  full 
value  of  blood  cells,  while  others  insist  they  are  the  nuclear 
remains  of  destroyed  leukocytes.  There  are  about  635,000 


40  THE  BLOOD 

to  one  cubic  millimeter  of  blood.  As  to  their  function  little 
is  known.  Some  claim  they  play  an  important  part  in  the 
coagulation  of  the  blood.  Nothing  definite  is  known  of  their 
origin. 

The  Coagulation  of  the  Blood. 

When  blood  is  allowed  to  stand  after  being  shed  it  rapidly 
becomes  more  viscous  and  later  sets  into  a  firm  jelly. 
Later,  as  the  fibrin  contracts,  a  clear  straw  colored  fluid,  the 
serum,  is  set  free.  The  formation  of  fibrin  is  the  essential 
factor  in  coagulation.  It  is  contained  in  the  plasma  in  the 
form  of  fibrinogen. 

The  relation  of  plasma,  serum  and  clot  is  shown  by  the  fol- 
lowing table  : 


Plasma 

Blood  \  dot 

Corpuscles 


CHAPTER  VI. 
THE    CIRCULATION    OF   THE    BLOOD. 

General. — We  have  seen  that  the  composition  of  the  blood 
fits  it  for  its  function  of  carrying  foodstuffs  to  the  tissues 
and  removing  the  products  of  combustion ;  but,  for  the  blood 
to  exercise  these  offices,  it  is  necessary  that  it  be  in  communi- 
cation wi'th  the  outside  world  and  the  tissues.  The  move- 
ment it  makes  through  its  network  of  vessels  in  order  to 
carry  products  from  the  exterior  to  the  interior  and  from 
the  interior  to  the  exterior  is  what  is  meant  by  circulation. 

Pulmonary  and  Systemic  Circulation. — Two  systems  of 
circulation  are  generally  distinguished.  The  first  is  the  pul- 
monary, and  is  the  circulation  of  the  blood  through  the  lungs 
in  order  to  get  rid  of  carbon  dioxide  and  to  get  a  fresh  sup- 
ply of  oxygen  by  aeration.  The  second  is  the  systemic  and 
is  the  circulation  through  the  great  masses  of  body  tissue  in 
order,  by  means  of  the  lymph,  to  supply  the  tissues  with 
different  solid,  liquid,  and  gaseous  nutritive  material  and 
take  from  the  tissues  the  products  no  longer  needed  but 
which  must  be  eliminated.  These  systems  are  also  called 
respectively  the  lesser  and  greater  circulation. 

Discovery. — The  circulation  of  the  blood  was  an  unknown 
fact  up  to  1628  when  the  discovery  of  its  movements  was 
made  and  proved  by  Sir  iWilliam  Harvey,  an  English  physi- 
cian prominent  in  his  time  and  now  famous  for  this  dis- 
covery. 

The  Circulatory  Apparatus. — The  blood  circulates  through 
a  series  of  closed  tubes  known  as  blood-vessels,  which  divide 
up,  ramify,  and  go  to  all  parts  of  the  body.  These  vary  from 
large,  macroscopic  vessels  to  tiny,  little,  hair-like  tubes,  the 
capillaries,  which  cannot  be  seen  with  the  naked  eye. 

41 


42  THE  CIRCULATION   OF  THE  BLOOD 

The  central  organ  of  the  circulatory  system  is  the  heart. 
From  this  lead  off  the  arteries,  these  in  turn  connect  with  the 
capillaries,  and  these  with  the  veins,  which  lead  back  to  the 
heart. 


I.  THE  HEART. 

The  heart  is  a  hollow,  muscular  organ  divided  by  a  mus- 
cular septum  into  two  distinct  compartments  designated  for 
convenience,  the  right  and  left  heart.  The  right  side,  and 
similarly  the  left,  is  divided  by  a  muscular  septum  into  two 
chambers,  the  upper  called  the  auricle  and  the  lower  the  ven- 
tricle. There  is  an  opening  between  the  right  auricle  and 
the  right  ventricle  and  one  between  the  left  auricle  and  the 
left  ventricle  and  each  opening  is  guarded  and  can  be  closed 
by  a  thin  membranous  flap  called  a  valve. 

Situation. — The  heart  is  located  in  the  thoracic  cavity  be- 
hind the  sternum.  It  is  placed  in  a  diagonal  position  and  its 
base  is  in  the  middle  line  and  looks  backward,  upward,  and 
to  the  right.  Its  apex  is  three  inches  to  the  left  of  the  median 
line,  a  half  inch  internal  to  the  nipple,  and  in  the  fifth  inter- 
costal space. 

Covering  and  Lining. — A  serous  sac,  called  the  pericar- 
dium, covers  the  heart.  It  hugs  the  muscle  of  the  heart 
closely,  completely  enveloping  the  organ,  then  turns  back  on 
itself  leaving  a  space  between  the  outer  layer  and  the  layer 
next  to  the  muscle.  In  this  space  is  a  fluid  which  acts  as  a 
lubricant. 

The  heart  is  lined  by  a  membrane  called  the  endocardium, 
which  is  composed  of  endothelial  tissue. 

Structure. — The  muscle  of  the  heart  is  striated,  but  con- 
trary to  the  usual  rule,  is  involuntary  in  its  action.  The  mus- 
cle fibers  run  circularly,  obliquely,  and  some  in  the  form  of 
the  figure  eight,  thus  giving  the  power  to  contract  and  pump 
the  blood  on  into  the  circulation. 


THE  HEART  43 

Contraction. — The  physiological  contraction  of  the  car- 
diac muscle  is  called  systole,  the  relaxation  is  called  diastole. 
The  contraction  of  the  heart  starts  at  the  mouth  of  the 
veins  and,  with  a  uniform  rhythm  glides  along  through  the 
auricles  and  along  to  the  ventricles,  each  part  relaxing  as  the 
rhythmic  contraction  passes  on.  The  whole  time  of  contrac- 


OIASTOLE 
OF 

UR1CLE&VENTRICLE. 


FIG.  22. — Scheme  of  cardiac  cycle. 

The  inner  circle  shows  the  events  which  occur  within  the  heart;  the  outer  the 
relation  of  the  sounds  and  pauses  to  these  events.  (Kirkes  after  Sharpey  and 
Gairdner.) 

tion,  from 'one  beginning  in  the  veins  to  another  beginning, 
is  called  the  cardiac  cycle.  It  lasts  about  .86  second. 

The  cycle  may  be  divided  thus :  the  auricles  contract  (sys- 
tole) and  ventricles  are  relaxed  (diastole)  which  occupies 
.16  second;  the  ventricles  contract  (systole)  and  the  auricles 
are  relaxed  (diastole)  and  this  occupies  .3  second;  both  au- 
ricles and  ventricles  then  rest  and  this  occupies  .4  second. 

Number  of  Beats. — In  an  adult  the  heart  beats  on  an  aver- 
age of  72  times  per  minute,  in  children  it  is  higher.  The  fre- 
quency of 'beat  is  influenced  by  age,  sex,  disease,  drugs,  phy- 
sical causes  and  digestion. 


44  THE  CIRCULATION  OF  THE  BLOOD 

Valves  and  Openings. 

Right  Auricle. — Leading  off  from  the  right  auricle  anter- 
iorly and  superiorly  is  a  sinus  that  bears  the  name  of  the 
auricular  appendix.  It  is  a  little  hollow  pouch  capable  of 
distention  with  blood. 

Opening  into  the  right  auricle  we  find  the  coronary  veins, 
the  two  venae  cavae,  and  the  auriculo-ventricular  opening. 
Guarding  these  openings  are  valves  to  prevent  the  backward 
flow  of  the  blood  current. 

Right  Ventricle. — Opening  into  the  right  ventricle  are  the 
pulmonary  artery  and  the  right  auricle. 

The  tricuspid  valve  guards  the  auriculo-ventricular  open- 
ing. It  is  composed  of  three  triangular  shaped  membranes 
attached  to  the  base  of  the  circumference  of  the  opening  and 
the  apices  of  the  triangles  coming  together  when  closed. 

The  semi-lunar  valves  guard  the  pulmonary  .opening. 
They  are  three  entirely  separate  segments  of  semi-lunar 
shape  and  are  attached  by  their  long  curved  margins  to  the 
circumference  of  the  artery  just  where  it  springs  from  the 
muscular  substance  of  the  ventricles. 

Left  Auricle. — Like  the  right  auricle,  this  cavity  has  a 
small  sinus  leading  off  from  it  anteriorly  and  superiorly— 
the  auricular  appendix.  The  openings  into  the  left  auricle 
are  the  four  pulmonary  veins  and  the  left  ventricle. 

Left  Ventricle. — This  ventricle  has  the  thickest  walls  and 
does  the  most  work  of  any  of  the  chambers  of  the  heart,  be- 
cause it  forces  the  fresh  arterial  blood  out  into  the  aorta  and 
thence  through  the  entire  systemic  circulation. 

The  aorta  and  the  left  auricle  open  into  •  this  ventricle. 
The  aortic  semi-lunar  valves  guard  the  aortic  opening.  They 
are  three  distinct  semi-lunar  shaped  membranes  to  close  the 
aortic  opening  at  the  end  of  the  systole.  The  mitral  or  bi- 
cuspid valve  closes  the  left  auriculo-ventricular  opening.  It 
is  somewhat  like  the  tricuspid  except  that  it  has'  only  two 
flaps  instead  of  three. 


VALVES   AND   OPENINGS  45 

Functions  of  Valves. — The  valves  are  arranged  at  the 
openings  of  the  different  chambers  of  the  heart  so  the  blood 
will  be  forced  in  a  constant  direction.  When  the  auricles 
are  at  systole  the  auriculo-ventricular  valves  are  open  thus 
letting  the  flow  of  blood  go  from  auricles  to  ventricles ;  but 
as  soon  as  auricular  diastole  and  ventricular  systole  begin 
these  valves  shut  and  the  blood  is  kept  from  flowing  back- 
ward into  the  auricles.  Then  the  semi-lunar  valves  are  open 
and  the  blood  is  forced  into  the  aorta  and  pulmonary  artery. 
When  ventricular  diastole  begins  these  semi-lunar  valves  are 
closed  and  thus  blood  is  prevented  from  running  back  into 
the  heart  from  the  arteries. 

Work  of  the  Heart. — The  work  done  by  the  heart  is  equal 
to  the  weight  of  a  column  of  blood  multiplied  by  the  height 
or  distance  to  which  this  column  is  carried  by  the  heart  force. 
The  column  of  blood  is  that  amount  that  is  sent  by  a  single 
contraction  of  the  heart  and  the  height  to  which  it  is  carried 
is  equal  to  the  pressure  in  the  aorta  and  pulmonary  arteries. 

The  amount  of  blood  thrown  into  the  aorta  at  each  con- 
traction of  the  ventricles  weighs  about  87  grams  (about  3 
oz.)  and  the  height  to  which  it  is  forced  is  about  1.5  meters 
or  5  feet  in  man. 

In  estimating  the  work  of  a  machine  the  English  express 
the  result  in  foot  pounds.  The  French  in  grammetres.  A 
foot  pound  is  the  energy  expended  in  raising  a  unit  weight 
(i  Ib.)  through  a  unit  distance  (i  ft.).  A  grammetre  is  the 
force  expended  in  raising  one  gram  one  meter.  Thus  the 
work  of  the  left  ventricle  at  each  contraction  is  130.5  gram- 
metres  (or  15  foot  pounds).  Add  45  grammetres  as  the 
work  done  by  the  right  ventricle  in  contracting.  If  the 
heart  beats  72  times  per  minute  it  will,  in  twenty-four  hours, 
do  18,000  kilogramme-metres  of  work. 

Sounds  of  the  Heart. — Listening  to  the  heart's  action 
through  the  thoracic  wall  we  hear  two  distinct  sounds.  The 
first  is  a  slightly  elongated  sound  and  comes  immediately 
after  the  beat  of  the  radial  pulse.  It  is  characterized  by  the 


46  THE  CIRCULATION  OF  THE  BLOOD 

syllable  lub.  The  cause  of  this  sound  is  supposedly  the 
closure  of  the  auriculo-ventricular  valves  combined  with 
the  sound  made  by  the  contracting  muscle.  It  can  best  be 
heard  over  the  apex  of  the  heart. 

The  second  sound  is  shorter  and  sharper  than  the  first  and 
is  heard  just  before  the  impulse  of  the  radial  pulse.  It  is 
characterized  by  the  shorter  syllable  dup. 

The  cause  of  this  sound  is  supposedly  the  closure  of  the 
aortic  semi-lunar  valves  along  with  those  of  the  pulmonary 
artery.  It  is  best  heard  in  the  right  second  intercostal 
space,  as  the  aortic  current  transmits  it. 

Certain  diseases  affect  the  heart  valves  and  the  sounds 
then  depart  from  the  normal.  Thus  it  is  of  importance  to 
know  the  cause  and  sound  of  the  normal  vibrations  so  as  to 
detect  the  diseased  conditions. 

Heart  Innervation. — The  nerves  that  inhibit  the  action  of 
the  heart  are  the  two  vagi;  cutting  these  results  in  an  in- 
crease of  the  frequency  of  the  heart  beats. 

The  nerves  that  accelerate  the  action  of  the  heart  are  the 
nervi  accelerantes,  which  are  branches  of  the  sympathetic 
system.  Stimulation  of  these  causes  increase  in  force 
and  frequency  of  heart  beats. 

II.    CIRCULATION  IN  BLOOD-VESSELS. 

Taking  the  heart  as  a  central  station  for  supplying  force, 
we  find  the  blood  current  constantly  going  from  a  place  of 
higher  pressure  to  a  place  of  lower  pressure. 

The  highest  pressure  is  in  the  muscular  center,  the  heart. 
Blood-vessels  connect  with  both  auricles  and  ventricles. 
Those  connecting  with  the  ventricles  and  carrying  blood 
away  from  the  heart  are  called  arteries  and  the  pressure  in 
these  is  high,  but  lower  than  in  the  heart.  Those  vessels 
connecting  with  the  auricles  and  carrying  blood  back  to  the 
heart  are  called  veins  and  the  pressure  is  lowest  of  all  in 
these. 


STRUCTURE  OF  THE  BLOOD-VESSELS  47. 

The  minute  vessels  that  connect  the  arteries  and  veins  and 
collect  waste  from  and  supply  nutritive  material  to  the  lymph 
stream  are  called  capillaries.  The  pressure  in  these  is  lower 
than  in  the  arteries  but  higher  than  in  the  veins. 

The  blood  is  thus  kept  in  motion,  constantly  going  from 
place  of  higher  to  lower  pressure. 

The  completed  circulation  is  thus : — 

(Beginning  with  the  right  auricle  of  the  heart.)  The  two 
venae  cavae  pour  venous  blood  into  the 
right  auricle  and  it  in  turn  empties  its 
contents  into  the  right  ventricle.  From 
here  the  blood  is  driven  into  the  pulmon-  -pic.  23.— Aor- 
ary  artery  (carrying  venous  blood)  to  be  tic  regurgitation. 
aerated  in  the  lungs.  From  the  lungs  it  (Greene.} 
comes  by  pulmonary  veins  (carrying  arterial  blood)  to  the 
left  auricle.  This  is  the  lesser  or  pulmonary  circulation. 

From  the  left  auricle  the  blood  goes  into  the  left  ventricle 
and  from  here  it  is  forced  into  the  aorta  and  thus  into  the 
systemic  arteries,  then  through  the  capillaries  to  the  veins 
and  back  by  means  of  the  venae  cavae  into  the  right  auricle. 

The  complete  cycle  in  man  takes  about  twenty-two  sec- 
onds. 

STRUCTURE  OF  THE  BLOOD-VESSELS. 

Arteries. — The  arteries  have  three  coats :  ( i )  the  external 
coat  called  the  tunica  adventitia,  which  is  composed  of 
fibrous  tissue  with  a  little  plain  muscular  tissue,  (2)  middle 
coat  or  tunica  media,  composed  of  yellow,  elastic  tissue,  and 
(3)  the  inner  coat  or  tunica  intima,  composed  of  endothe- 
lium. 

Veins. — The  veins  also  have  three  coats,  the  external,  the 
middle  and  internal,  as  the  arteries ;  but  the  middle  coat  is 
composed  chiefly  of  inelastic,  fibrous  tissue.  Thus  the  veins 
lack  the  elasticity  and  contractility  given  to  the  arteries  by 
the  middle  coat. 


48 


THE  CIRCULATION  OF  THE  BLOOD 


The  Capillaries. — As  the  arteries  get  smaller  we  find  them 
still  composed  of  the  three  above  named  coats.  Finally, 
though,  in  the  minutest  vessels  we  find  only  the  innermost 

layer  remaining.  These  one- 
coated  vessels  are  the  capillaries, 
and  they  have  only  one  layer  of 
endothelial  cells  on  a  basement 
membrane.  This  is  in  order  to 
render  possible  the  interchange 
of  material  between  the  blood  cur- 
rent and  the  lymph  stream,  so  the 
tissues  may  be  nourished  and  the 
waste  products  removed. 

IMPORTANCE  OF  ARTERIAL 
ELASTICITY. 


If  an  amount  of  fluid  corre- 
sponding to  that  of  the  "pulse 
volume"  be  suddenly  injected  into 
the  end  of  a  rubber  tube  already 
distended  with  liquid,  the  tube 
will  be  further  distended  by  the 
liquid  injected,  but  if  a  like 
amount  of  fluid  be  allowed  to  es- 
cape at  the  other  end  the  tube  will 
resume  its  original  caliber.  Thus 
the  pulse  -volume  enters  -with 
much  force  (the  aorta  or  pulmon- 
ary artery;  the  artery  is  very 
elastic  and  expands  under  this  in- 
fluence, but  immediately  recoils 
with  a  great  pressure  on  the  con- 
tents. The  pressure  tends  to 
vessel  in  both  directions,  but  its 
is  effectually  prevented  by  the 


FIG.     24. — Scheme     of     the 
circulation. 

a,  right,  b,  left,  auricle;  A, 
right,  B,  left,  ventricle;  i,  pul- 
monary artery;  2,  aorta;  i,  area 
of  pulmonary,  K,  area  of  syste- 
mic, circulation;  o,  the  superior 
vena  cava;  G,  area  supplying  the 
inferior  vena  cava;  u;  d,  d,  in- 
testine; m,  mesenteric  artery;  q, 
portal  vein;  L,  liver;  h,  hepatic 
vein.  (Landois.) 

force  the  blood  along  the 
return   into   the   ventricle 


IMPORTANCE   OF   ARTERIAL    ELASTICITY  49 

closure  of  the  semi-lunar  valves.  Consequently  it  can  go 
only  toward  the  periphery. 

Now  it  is  evident  that  the  flow  in  the  beginning  of  the 
aorta  is  intermittent ;  but  it  is  found  that,  in  vessels  as 
large  as  the  carotids  the  flow  has  resumed  a  remittent  char- 
acter. The  smaller  the  vessel  the  nearer  the  flow  becomes 
continuous  until  this  condition  is  established  in  the  capil- 
laries. 

It  is  the  elastic  coat  of  the  arteries  which  allows  them 


FIG.   25. — Transverse   section  of  part  of  the  wall  of  the  posterior 
tibial  artery.    (Man.)    (From   Yeo  after  Shafer.} 

a,  endothelium  lining  the  vessel,  appearing  thicker  than  natural  from  the  con- 
traction of  they  outer  coats;  b,  the  elastic  layer  of  the  intima;  c,  middle  coat 
composed  of  muscle  fibers  and  elastic  tissue;  d,  outer  coat  consisting  chiefly  of 
white  fibrous  tissue. 

to  expand  and  contract,  thus  forcing  the  contents  onward. 
Furthermore  it  is  this  elasticity  that  causes  the  intermit- 
tent and  remittent  flow  to  become  continuous.  So  the  func- 
tion of  the  elastic  coat  is  two-fold ;  first,  it  forces  the  blood 
current  continuously  toward  the  periphery,  and  second,  it  is 
chiefly  the  cause  of  the  change  from  an  intermittent  flow 
to  a  constant  flow,  which  is  of  so  much  importance  in  the 
capillaries 

Rate  of  Flow. — The  velocity  of  the  blood  current  is  equal 
to  the  volume  flowing  through  a  determined  section  in  one 
second  divided  by  the  cross  section.  The  rate  is  determined 
by  the  pressure,  the  friction  in  the  vessels,  and  the  cross  sec- 
tion of  the  vessels. 


5O  THE  CIRCULATION  OF  THE  BLOOD 

The  combined  cross  section  of  the  capillaries  is  greater 
than  the  combined  cross  section  of  the  arteries  or  the  veins, 
so  the  rate  of  flow  must  be  greater  in  the  arteries  and  veins 
than  in  the  capillaries.  The  friction  is  greater  in  the  smaller 
vessels  than  in  the  larger  which  retards  the  flow.  The 
pressure  is  greater  in  the  arteries  than  in  the  capillaries 
and  veins.  From  these  facts  it  is  evident  that  the  velocity 
is  greater  in  the  arteries  than  in  the  capillaries  and  veins, 
but  increases  in  the  veins  as  compared  to  the  capillaries. 

In  the  large  arteries  the  rate  is  200-400  mm.  per  second, 
in  the  capillaries,  6-8  mm.  and  in  the  large  veins  it  is  but 
little  less  than  in  the  arteries. 

Valves  in  the  Veins. — At  frequent  intervals  in  the  course 
of  the  veins  are  found  small  folds  of  membrane  protruding 
into  the  lumen  of  the  vessels.  The  flow  of  the  blood  in  the 
veins  is  more  sluggish  than  in  the  arteries,  because,  as  we 
have  seen,  the  pressure  lessens  in  the  veins  while  gravity 
and  friction  tend  to  cause  a  stoppage.  These  protruding  folds 
of  the  endothelial  membrane  or  valves  found  in  the  veins 
aid  in  the  circulation  by  overcoming  gravity  and  preventing 
a  backward  flow  of  blood,  by  holding  the  blood  until  a 
fresh  impulse  can  impel  it  forward.  They  are  found  in 
pairs  and  are  most  abundant  in  the  veins  of  the  extremities 
where  gravity  impedes  the  onward  flow  of  the  current. 

Capillary  Importance. — The  capillaries  are  the  smallest 
blood-vessels  and  the  most  important  as  to  function.  Being 
of  only  one  thickness  of  endothelium  and  in  direct  com- 
munication with  the  lymph  flow,  we  can  readily  see  that  the 
food  products  brought  by  the  arterial  blood  can  be  ex- 
changed here  for  waste  brought  by  the  lymph.  The  flow  in 
the  capillaries  is  constant,  as  we  have  already  sfeen.  We 
can  understand  the  importance  of  this  when  we  take  into 
consideration  the  rapidity  with  which  the  tissues  use  oxy- 
gen, the  necessity  of  a  constant  supply,  and  the  importance 
of  removing  the  carbon  dioxide  poisons. 

Innervation  of  Vessels. — The  blood-vessels  are  controlled 


INNERVATION  OF  VESSELS  51 

by  the  sympathetic  nervous  system  by  means  of  the  vaso-mo- 
tor  nerves.  These  are  composed  of  the  vaso-constrictors 
which  cause  the  vessels  to  contract,  and  the  vaso-dilators 
which  cause  them  to  dilate.  The  entire  physiological  distribu- 
tion of  blood  is  regulated  by  the  vaso-motor  system  of  nerves. 
It  is  by  their  means  that  the  blood  is  increased  to  any  part 
of  the  body  where  physiological  activity  is  going  on,  as 


FIG.  26. 

A,  vein  with  valves  open.     B,  with  valves  closed;  stream  of  blood  passing  off  by 
lateral  channel.     (Kirkes  after  Dalton.) 

when  the  gastro-intestinal  tract  is  active  during  digestion, 
when  a  muscle  is  in  motion,  or  a  gland  in  activity.  Paraly- 
sis of  (the  vaso-constrictors  causes  blushing,  paralysis  of  the 
dilators  causes  pallor  as  from  fright.  Outside  influences 
will  cause  the  constrictors  to  act,  as  cold ;  while  alcohol  will 
cause  the  dilators  to  act  and  paralyzes  the  constrictors. 

The  chief  vaso-motor  center  is  in  the  medulla  oblongata, 
while  subordinate  centers  exist  in  the  cord.  The  vaso-motor 
fibers  reaching  the  vessels  proceed  from  ganglia  in  the  sym- 


52  THE  CIRCULATION  OF  THE  BLOOD 

pathetic  system,  but  these  ganglia  are  influenced  by  the  cells 
in  the  vaso-motor  center. 

Amount  of  Blood  Important. — When  there  is  a  small  loss 
of  blood  from  a  slight  injury  the  entire  vascular  system  con- 
tracts and  the  current  supplying  this  diminished  area  is 
sufficient;  but  at  other  times  the  loss  of  blood  is  so  great  that 
the  amount  remaining  is  not  sufficient  to  carry  on  a  complete 
circulation.  Unless  remedied  this  results  in  death.  In  such 


FIG.  27. — Capillaries. 

The  outlines  of  the  nucleated  endothelial  cells  with  the  cement  blackened  by  the 
action  of  silver  nitrate.     (Landois.) 


cases  of  great  loss  the  deficit  may  be  supplied  by  a  normal 
salt  solution,  thus  giving  an  amount  of  fluid  sufficient  to 
maintain  the  heart  action.  But  in  cases  where  as  much  as 
two-thirds  of  the  blood  is  lost,  the  injection  of  fluid  does  no 
good.  The  amount  of  fluid  necessary  to  cause  the  heart's 
action  to  continue  may  be  supplied,  but  the  amount  of  hemo- 
globin necessary  for  life  is  lost  and  this  cannot  be  sup- 
plied. Asphyxiation  is  the  result. 


PULSE 


53 


Pulse. — If  a  finger  be  placed  on  any  artery  in  the  body 
there  will  be  transmitted  to  it  a  perceptible  impulse.  This 
impulse  is  what  is  called  the  pulse.  It  is  caused  by  the  force 
of  the  heart's  action  against  the  elastic  arterial  wall, 'and  the 


FIG.  28. — Interior  of  right  auricle    and    ventricle    exposed    by    the 
removal  of  a  part  of  their  walls.    (From  Yeo  after  Allen- 
Thompson.} 

i,  superior  vena  cava;  2,  inferior  vena  cava;  2',  hepatic  veins;  3,  3',  3",  inner 
wall  of  right  auricle;  4,  4,  cavity  of  right  ventricle;  4',  papillary  muscle;  5, 
5',  5",  flaps  of  tricuspid  valve;  6,  pulmonary  artery  in  the  wall  of  which_  a 
window  has  been  cut;  7,  on  aorta  near  the  ductu's  arteriosus;  8,  9,  aorta  and  its 
branches;  10,  u,  left  auricle  and  ventricle. 

subsequent  contraction  of  this  wall  against  the  current  it 
contains. 


54 


THE  CIRCULATION  OF  THE  BLOOD 


The  impulse  is  an  index  to  the  condition  of  the  circulation. 
Its  frequency  normally  in  an  adult  is  about  72  times  per 


FIG.  29. — The  left  auricle  and  ventricle  opened  and  part  of  their 

walls  removed  to  show  their  cavities.     (From  Yeo 

after  Allen  Thompson.) 

i,  right  pulmonary  vein  cut  short;  i',  cavity  of  left  auricle;  3,  3",  thick  wall 
of  left  ventricle;  4,  portion  of  the  same  with  papillary  muscle  attached;  5,  the 
other  papillary  muscles;  6,  6',  the  segments  of  the  mitral  valve,  7,  in  aorta  is 
placed  over  the  semi-lunar  valves. 

minute,  in  children  it  is  higher,  and  it  is  more  frequent  in 
woman  than  in  man.  Its  frequency  is  affected  by  age,  sex, 
exercise,  disease,  drugs,  and  psychical  causes,  as  fear,  sor- 


PULSE 


55 


row,  etc.  We  feel  the  pulse  to  learn  several  things: — (i) 
Its  frequency,  which  tells  how  many  times  the  heart  is  beat- 
ing. 

(2)   Its  tension,  which  is  the  state  of  the  arterial  walls 


FIG.  30. — Portion  of  the  wall  of  ventricle. 

d,  d',  and  aorta,  a,  b,  c,  showing  attachments  of  one  flap  of  mitral  and  the 
aortic  valves;  h  and  g,  papillary  muscles;  e,  e'  and  f,  attachment  of  the  tendi- 
nous cords.  (From  Yeo  after  Allen  Thompson.) 

and  is  the  resistance  offered  in  peripheral  vessels.  We 
judge  the  tension  by  the  force  necessary  to  obliterate  the 
impulse. 


50  THE  CIRCULATION  OF  THE  BLOOD 

(3)  Regularity,  which  tells  whether  the  heart  is  regular 
in  either  its  force  or  rhythm. 

(4)  Its  strength,  which  tells  as  to  the  force  with  which 
the  heart  is  acting. 

(5)  Its  length,  whether  the  beat  is  long  or  slow  and  con- 
tinuous. 

(6)  The  condition  of  the  vessel  wall,  whether  sclerotic 


FIG.  31. — Dudgeon  sphygmograph. 


or  not.  In  the  study  of  the  pulse  an  instrument  called  the 
sphygmograph  is  used,  which  receives  the  impulse  from  a 
beating  artery  and  transmits  it  by  means  of  a  finely  ad- 
justed lever  to  a  smoked  surface  of  paper.  Thus  a  graphic 
representation  of  the  impulse  .is  given,  the  height  to  which 
the  writing  end  of  the  lever  goes  denoting  the  force  of  the 
impulse  of  the  heart  beat  at  the  time  of  the  writing. 


THE  LYMPH  57 

THE  LYMPH. 

The  lymph  is  a  clear  colorless  fluid  contained  in  the  lym- 
phatic vessels  and  tissue  spaces.  It  resembles  plasma  in  gen- 
eral appearance  and  does  not  differ  greatly  from  it  in  com- 
position. 

The  Lymph  Vessels.— These  vessels  originate  in  at  least 
three  different  ways.  ( i )  All  cells  may  be  said  to  be  bathed 
in  lymph,  being  surrounded  by  that  fluid  lying  in  the  irregu- 
larly shaped  spaces  between  them.  These  spaces  communi- 
cate with  each  other  and  finally  converge  to  the  lymph  ca- 
pillaries. The  intervals  are  called  the  "extravascular  lymph 
spaces!'  (2)  In  certain  situations,  particularly  in  the  ner- 
vous centers,  the  small  blood-vessels  are  completely  sur- 
rounded by  and  included  in  larger  tubes,  the  "perivascular 
lymph  canals."  These  likewise  pass  on  to  the  lymph  capil- 
laries proper.  ( 3 )  The  large  serous  cavities,  like  those  lined 
by  the  peritoneum,  pleura,  tunica  vaginalis,  etc.,  have  large 
numbers  'of  lymphatic  radicles  opening  abruptly  into  them, 
or  rather  originating  from  them,  and  these  may  be  consid- 
ered as  great  extravascular  lymph  spaces. 

The  course  of  the  lymph  is  from  the  tissues  to  the  sub- 
clavian  veins,  where  it  enters  the  vascular  circulation.  The 
lymphatic  vessels  from  the  right  arm  and  the  right  side 
of  the  face,  head  and  chest  converge  to  form  the  ductus  lym- 
phaticus  dexter,  which  enters  the  right  subclavian  vein  at  its 
junction  with  the  internal  jugular.  The  lymphatics  from  all 
other  parts  of  the  body  converge  to  form  the  thoracic  duct, 
which  enters  the  left  subclavian  vein  at  its  junction  with  the 
internal  jugular.  The  thoracic  duct  begins  by  a  dilated 
pouch  lying  upon  the  second  lumbar  vertebra.  This  pouch 
receives  the  lymphatic  branches  which  have  converged  from 
the  lacteals,  and  is  called  the  receptaculum  chyli.  The  lac- 
teals  pass  through  the  mesenteric  lymphatic  glands  on  their 
way  to  the  receptaculum  chyli. 

The  distribution  of  the  lymphatics  needs  no  comment  when 


THE  CIRCULATION  OF  THE  BLOOD 


FIG.  32. — Diagram  showing  the  course  of  the  main  trunks  of  the 
absorbent  system. 

The  lymphatics  of  lower  extremities,  D,  meet  the  lacteals  of  intestines,  LAC, 
at  the  receptaculum  chyli,  R.C.,  where  the  thoracic  duct  begins.  The  superficial 
vessels  are  shown  in  the  diagram  on  the  right  arm  and  leg,  S,  and  the  deeper 
ones  on  the  left  arm,  D.  The  glands  are  here  and  there  shown  in  groups.  The 
small  right  duct  opens  into  the  veins  on  the  right  side.  The  thoracic  duct  opens 
into  the  union  of  the  great  veins  of  the  left  side  of  the  neck,  T.  (Yco.) 


THE  LYMPHATIC  GLANDS  59 

it  is  known  that  they  receive  the  plasma  which  has  been 
passed  out  of  the  vascular  capillaries  and  thus  collect  fluid 
from  well-nigh  every  tissue  in  the  body. 

The  structure  of  the  lymph- vessels  is  quite  similar  to  that 
of  the  veins,  though  they  are  more  delicate.  The  lymph 
capillaries  probably  contain  only  a  single  coat  like  the  venous 
capillaries.  In  the  large  vessels  this  thin  endothelial  coat  is 
supplemented  by  conne'ctive  tissue  fibers  together  with  some 
elastic  and  non-striated  muscle  fibers.  They  are  very  abun- 
dantly supplied  with  valves  which  operate  in  the  same  way 
as  the  venous  valves.  The  vessel  wall  is  quite  elastic  and  has 
some  contractile  power. 

Lymphatic  Glands. — 'All  the  lymphatics  pass  through  one 
or  more  lymphatic  glands  on  their  way  to  the  Larger  trunks. 
These  bodies  are  not  true  glands.  Their  structure  is  adenoid. 
There  are  some  six  or  seven  hundred  in  the  body,  varying  in 
size  from  a  pinhead  to  a  large  bean.  The  superficial  glands 
are  especially  abundant  about  the  groin,  axilla,  neck  and 
other  flexures.  The  deep  glands  are  most  numerous  about 
the  great  vessels.  The  mesenteric  glands  are  found  between 
the  folds  of  the  mesentery. 

The  lymphatic  glands  are  of  irregular  shape  and  contain 
within  their  substance  large  numbers  of  lymph  spaces  or 
canals  through  which  the  incoming  lymph  must  pass.  The 
vasa  efferentia  are  usually  fewer  in  number  and  larger  in 
size  than  the  vasa  afferentia.  The  current  must  be  consid- 
erably delayed  in  the  glands.  They  are  concerned  in  the  pro- 
duction of  leucocytes,  while  their  retention  of  toxic  materials 
— even  to  their  own  hurt — is  a  common  pathological  occur- 
rence. 

Properties  and  Composition  of  Lymph. — Lymph  is  a  com- 
paratively clear  liquid  containing  leucocytes.  After  meals 
the  color  becomes  whitish  from  the  admixture  of  chyle,  and 
numerous  fat  droplets  are  present.  Neither  red  corpuscles 
nor  platelets  are  found  in  lymph  except  accidentally.  The 
specific  gravity  is  lower  than  that  of  the  blood.  Lymph 


60  THE  CIRCULATION  OF  THE  BLOOD 

coagulates  when  drawn,  since  the  fibrin  factors  are  present ; 
but  the  process  is  less  prompt  and  the  clot  is  less  firm  than 
in  the  case  of  blood. 

In  order  to  form  an  idea  as  to  the  constituents  of  lymph 
it  is  only  necessary  'to  say  that  its  ultimate  origin  is  the  blood 
plasma,  except  in  so  far  as  its  composition  is  changed  during 
digestion.  The  plasma  makes  its  way  through  the  capillary 
walls  out  to  the  tissues  bringing  nourishment  to  them  and  re- 
moving waste  products  from  them.  In  thus  coming  in  con- 
tact with  the  tissues  the  plasma  finds  itself  in  the  extravas- 
cular  lymph  spaces  and  its  name  is  simply  changed  to  lymph. 
lit  thus  appears  that  lymph  may  enter  the  extravascular 
spaces  by  the  direct  passage  of  plasma  out  of  the  vessels  or 
by  being  excreted,  as  it  were,  from  the  tissue  cells. 

In  any  case  the  constituents  of  lymph  are  not  very  differ- 
ent from  those  of  plasma,  except,  of  course,  when  intestinal 
digestion  is  in  progress  and  chyle  is  introduced  into  the  lym- 
phatic circulation.  It  contains  the  three  plasma  proteids, 
urea,  fat,  lecithin,  cholesterin,  sugar  and  inorganic  salts. 
The  proteids  are  less  abundant  than  in  plasma,  as  might  be 
supposed  when  it  is  remembered  that  they  possess  little  os- 
motic power.  The  inorganic  saks  are  in  about  the  same 
proportion  in  both  fluids.  It  is  significant  that  the  amount 
of  urea  and  related  excrementitious  products  is  more  abun- 
dant in  lymph  than  in  plasma ;  their  source  is  the  destructive 
metabolism  going  on  in  the  cells  to  which  the  plasma  has  been 
supplied,  this  plasma  finding  its  way  back  as  lymph.  It  is  by 
no  means  certain,  however,  that  all  the  plasma  escaping  from 
the  capillaries  is  carried  away  by  the  lymphatic  system. 
Some  may  reenter  the  blood-vessels. 

There  is  no  unanimity  of  opinion  as  to  the  exact  method 
of  passage  of  plasma  through  the  capillary  walls  into  the 
lymph  spaces.  Some  maintain  that  the  phenomena  can  be 
explained  by  the  ordinary  physical  laws  of  diffusion,  filtra- 
tion and  osmosis  when  existing  conditions  of  pressure,  etc., 
are  taken  into  consideration.  Others  hold  that  these  laws  are 


THE  FLOW   OF   LYMPH  6 1 

insufficient  in  themselves  to  account  for  various  occurrences 
in  this  connection,  and  ascribe  to  the  capillary  endothelium 
some  active  secretory  power  governing,  or  at  least  influenc- 
ing, the  outward  passage  of  the  plasma. 

The  Flow  of  Lymph. — There  is  no  organ  corresponding  to 
the  heart  to  keep  the  lymph  current  in  motion.  The  main 
causes  for  its  direction  from  the  extravascular  spaces  toward 
the  veins  in  the  neck  is  the  degree  of  pressure  to  which  it  is 
subjected  in  those  spaces  as  compared  with  the  inferior,  or 
even  "negative,"  pressure  obtaining  near  the  terminations  of 
the  great  ducts.  It  is  known  that  at  all  times  the  venous 
pressure  in  the  subclavian  veins  is  low  and  that  it  may  even 
fall  below  the  atmospheric  pressure,  so  that  "suction"  is  ex- 
erted upon  the  lymphatic  ducts  where  they  enter  those  ves- 
sels. The  lymph  pressure  in  the  extravascular  spaces  is  esti- 
mated to  be  one-half  the  capillary  blood-pressure.  Friction 
and  gravity  (where  the  course  of  the  vessels  is  upward)  op- 
pose the  passage  of  the  fluid.  Consequently  it  accumulates 
in  the  spaces  and  in  the  smaller  lymphatics  until  the  pressure 
there  becomes  greater  than  the  resistance  of  these  forces, 
when  it  passes  onward.  Since  lymph  is  being  continually 
produced  this  superior  pressure  in  the  extravascular  spaces 
and  small  lymphatics  is  a  fairly  constant  factor  and  keeps  up 
a  correspondingly  constant  current. 

There  are  two  factors  which  are  accessory  to  this  peri- 
pheral pressure :  ( i )  Thoracic  aspiration  by  bringing  about 
negative  pressure  in  the  veins  in  and  near  the  chest  brings 
about  a  like  condition  in  the  tributary  lymphatic  ducts ;  fur- 
thermore, the  effect  of  aspiration  makes  itself  felt  directly 
upon  the  thoracic  duct  since  its  greatest  extent  is  in  the  tho- 
rax. (2)  The  valves  of  the  lymphatics  act  in  a  similar  man- 
ner to  those  of  the  veins  and  constitute  a  very  necessary 
factor  in  the  lymphatic  circulation.  Although  the  lymph 
flow  resembles  that  of  the  venous  blood,  it  is  less  regular  and 
more  sluggish,  but  probably  not  so  slow  as  might  be  sup- 
posed. Properly  colored  solutions  injected  into  the  blood 


62  THE  CIRCULATION  OF  THE  BLOOD 

have  been  demonstrated  in  the  lymph  of  the  thoracic  duct 
"in  from  four  to  seven  minutes." 

Lymph  and  Chyle. — It  is  scarcely  necessary  to  refer  to  the 
differences  between  these  two  fluids.  Chyle  is  the  intestinal 
lymph  during  digestion.  In  the  intervals  of  digestion  the 
contents  of  the  lacteals  do  not  differ  materially  from  lymph 
in  other  localities.  Chyle  has  a  whitish  milky  appearance  due 
to  the  presence  of  emulsified  and  saponified  fats.  Its  specific 
gravity  naturally  depends  largely  upon  the  amount  of  fat  in- 
gested, but  is  always  higher  than  that  of  ordinary  lymph  and 
lower  than  that  of  blood.  Not  only  is  there  more  fat  in  the 
chyle  than  in  lymph,  but  the  other  solids  are  also  increased. 
The  proteid  constituents  are  considerably  more  abundant. 
For  the  most  part  the  higher  specific  gravity  is  explained  by 
the  absorption  of  solids  in  solution  from  the  alimentary 
canal. 

Chyle  is  forced  out  of  the  lacteals  by  contraction  of  the 
non-striated  muscle  fibers  which  run  along  by  the  vessel. 
When  relaxation  of  the  fibers  occurs,  return  of  chyle  into 
the  lacteal  is  prevented  by  a  valve  at  the  base  of  the  villus. 


CHAPTER  VII. 
THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION. 

FOODS. 

IT  is  evident  that  all  the  tissues  of  the  body  are  continually 
undergoing  "physiological  wear" — that  the  materials  of 
which  they  are  intrinsically  composed  are  being  changed  into 
effete  matter  and  discharged  from  the  system.  This  is  a 
process  going  on  in  the  substance  of  every  cell  in  the  body, 
and  obviously,  for  these  cells  to  continue  to  live  and  func- 
tionate, there  must  be  a  continual  appropriation  of  new  mat- 
ter to  take  the  place  of  the  materials  which  have  served 
their  physiological  purpose,  and  are  of  no  further  value  to 
the  body.  This  supply  of  material  is  made  directly  to  the 
tissues  by  the  blood,  but  lest  this  fluid  be  impoverished,  it 
must  in  turn  be  furnished  with  an  approximate  constant 
quantity  of  nutritive  matter.  The  ultimate  source  of  that 
matter  is  in  the  food  which  we  eat.  However,  it  must  pass 
through  the  processes  of  digestion  and  absorption  before  it 
can  be  utilized  by  the  tissues.  This  conception  of  a  food 
must  be  understood  to  embrace  all  substances  contributing, 
either  directly  or  indirectly,  to  body  nutrition,  including, 
therefore,  the  oxygen  of  the  air  as  well  as  all  articles  usually 
classed  as  drinks. 

An  animal  whose  weight  remains  about  the  same  must  eat 
and  digest  a  certain  quantity  of  food  to  keep  up  the  body 
temperature,  to  supply  mechanical  energy,  and  to  repair  the 
wastes  which  are  continually  going  on  in  the  body.  An  ani- 
mal which  is  growing  and  increasing  in  weight  must  eat 
enough  not  only  to  supply  the  demands  just  mentioned,  but 
also  to  form  the  new  tissue. 

63 


64  THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

The  articles  we  eat,  besides  being  largely  insoluble,  differ 
very  materially  in  their  composition  from  any  substances 
found  as  parts  of  the  body  tissues.  Even  those  undigested 
substances  most  closely  resembling  living  tissue  will  no.1  be 
utilized  by  the  cells  when  presented  to  them  by  being  injected 
into  the  blood.  All  the  articles  which  we  use  for  food  must 
undergo  a  special  process,  called  digestion,  before  they  can 
be  absorbed  by  the  tissues. 

Seat  of  Hunger. — Food  is  taken  into  the  body  in  obedience 
to  an  expressed  want  on  the  part  of  the  system.  The  desire 
for  food — the  sensation  of  hunger — is  referred,  in  a  rather 
indefinite  way,  to  'the  stomach.  That  sensation  is  ordinarily 
satisfied  by  the  introduction  of  food  into  the  stomach.  How- 
ever, this  does  not  necessarily  mean  that  its  seat  is  in  that 
organ,  since  removal  of  the  stomach  by  no  means  prevents 
hunger.  .But,  if  nutritious  material  be  introduced  in  suffi- 
cient quantity  into  the  circulation,  as  by  rectal  enemata,  hun- 
ger is  relieved.  The  true  seat  of  this  sensation  is  undoubt- 
edly in  the  cells  themselves,  it  being  simply  a  call  from  them 
for  more  material  to  take  the  place  of  their  worn-out  con- 
stituents. 

Cold  weather  demands  an  increase  in  the  amount  of  food, 
as  also  do  physical  and  psychical  activity,  certain  drugs,  etc. 

Seat  of  Thirst. — The  demands  of  the  cells  for  water  is  re- 
ferred to  the  fauces  and  throat,  but  this  is  no  more  the  seat 
of  thirst  than  is  the  stomach  of  hunger.  The  taking  of 
water  into  the  mouth  alone  will  not  quench  thirst,  except  in 
so  far  as  absorption  may  take  place  from  its  mucous  mem- 
brane. But,  if  water  in  sufficient  amount  be  placed  into  the 
circulation  in  any  way  satisfaction  ensues.  Next  to  the  de- 
mand for  oxygen,  that  for  water  is  the  most  imperative 
which  comes  from  the  tissues;  that  is,  they  can  live  much 
longer  without  solid  food  than  without  water.  The  amount 
necessary  is  manifestly  subject  to  many  conditions,  such  as 
external  moisture  and  temperature,  exercise,  etc. 

Classification  of  Foods. — A  very  large  number  of  sub- 


FOODS  65 

stances  are  taken  into  the  alimentary  canal  as  food ;  but  ex- 
amination reveals  that  all  such  materials  contain  one  or  more 
of  a  very  few  classes  of  food  stuffs.  These  may  be  divided 
as  follows: 

I.  Water. 
II.  Inorganic  or  mineral  salts. 

III.  Carbohydrates. 

IV.  Fats. 

V.  Proteids. 

I.  Water  is  scarcely  looked  upon  as  food  in  the  common 
acceptation  of  the  term,  but  it  is  quite  as  necessary  to  cell  life 
as  any  of  the  other  classes.    It  is  found  in  all  foods  and  in 
all  tissues  and  fluids  of  the  body.    It  forms  about  70  per  cent, 
of  the  entire  body  weight  and  acts  as  a  solvent  upon  vari- 
ous ingredients  of  the  food,  liquefying  them  and  rendering 
them  capable  of  absorption. 

II.  The  mineral  salts  which  are  chiefly  necessary  for  nutri- 
tion are: 

Chlorides      "1 

Sujhater    f  Of  sodium  and  P°tassium- 

Carbonates 

Phosphates  V  Qf  ca]cium  and  esium 

Carbonates   J 

Of  these  salts,  sodium  chloride,  or  common  table-salt,  is  the 
most  important  and  abundant  in  the  foods  we  eat.  It  is 
present  in  nearly  all  the  tissues  and  fluids  of  the  body,  es- 
pecially the  blood.  Of  the  other  salts,  those  of  calcium  exist 
in  the  largest  quantity  in  the  body.  They  are  especially  im- 
portant on  account  of  the  part  they  play  in  the  formation  of 
the  bones,  teeth  and  cartilages.  The  remaining  salts  exist 
in  larger  or  smaller  quantities  in  the  tissues  and  fluids  of  the 
body. 

III.  The  carbohydrates  include  principally  the  starches 


66  THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

and  sugars.  They  are  of  definite  chemical  composition  con- 
taining carbon,  hydrogen  and  oxygen,  but  no  nitrogen.  The 
hydrogen  and  oxygen  which  they  contain  are  always  in  the 
proportion  to  form  water,  i.  e.,  two  atoms  of  hydrogen  to  one 
of  oxygen.  The  starches  are  found  chiefly  in  wheat,  corn, 
oats  and  other  grains ;  also  in  potatoes,  peas,  beans  and  in  the 
roots  and  stems  of  many  plants,  and  in  some  fruits.  Starch 
is  found  in  a  pure  state,  as  a  white  powder,  in  arrowroot  and 
corn-starch.  The  sugars  are  of  several  kinds,  the  principal 
being :  cane  sugar,  beet  sugar,  maple  sugar,  grape  sugar  which 
is  found  in  grapes,  peaches  and  other  fruits,  and  malt  sugar 
which  is  obtained  from  malt.  These  are  all  obtained  from 
vegetable  tissue,  however  a  few  are  found  in  or  formed  by 
the  animal  organisms,  as  glycogen,  dextrose  and  lactose. 
They  are  the  cheapest  foods  from  financial  and  digestive 
standpoints  and  constitute  the  main  bulk  of  articles  eaten. 
They  contain  more  oxygen  than  do  the  fats,  and  are  more 
easily  oxidized  and  converted  into  heat  and  muscular  energy. 
In  fact,  their  great  physiological  value  lies  in  the  ease  with 
which  they  are  burned  up  in  the  body.  They  furnish  the 
main  part  of  the  fuel  necessary  to  the  running  of  the  animal 
mechanism.  They  may  also  be  converted  into  fatty  tissue  by 
the  body. 

IV.  The  fats  are  ingested  with  both  animal  and  vegetable 
diets.  They  are  compounds  of  carbon,  hydrogen  and  oxy- 
gen. The  principal  fats  are  stearin,  palmatin,  margarin  and 
olein.  These  exist  in  varying  proportions  in  the  fat  of  ani- 
mals, in  the  various  vegetable  oils  and  in  milk,  butter,  lard 
and  in  other  foods  and  vegetable  substances.  The  fats  contain 
no  nitrogen,  and,  Jike  carbohydrates,  their  great  physiological 
value  lies  in  the  fact  that  they  are  destroyed  in  the  organism 
to  produce  energy,  whether  in  the  form  of  heat  or  muscu- 
lar exercise.  They  are  handled  and  converted  less  readily  by 
the  system  than  the  carbohydrates,  and  consequently  tax  the 
digestive  powers  more.  But  it  is  found  that,  weight  for 
weight,  they  are  the  more  efficient  in  the  production  of 


FOODS  67 

energy  than  are  the  carbohydrates.  They  also  furnish  fuel 
for  the  running  of  the  body  mechanism. 

V.  The  proteids  form  a  large  part  of  all  living  organisms 
and  are  absolutely  necessary  to  animal  life.  They  are  very 
stable  compounds  and  are  found  in  both  animal  and  vegetable 
foods.  They  contain  carbon,  hydrogen,  oxygen  and  nitro- 
gen, together  with,  usually,  a  small  quantity  of  sulphur  and 
phosphorus.  They  occur  in  the  form  of  casein  in  milk  and 
cheese,  myosin  and  syntonin  in  muscle,  vitellin  in  the  yolk  of 
eggs,  glutein  in  flour,  legumin  in  peas,  beans  and  lentils,  and 
in  some  other  forms.  Proteids  may  be  used  by  the  body  to 
produce  heat  and  energy,  but  being  more  stable  in  composi- 
tion than  carbohydrates  and  fats,  they  are  more  often  used 
to  build  up  tissue.  In  fact  the  proteids  are  absolutely  essen- 
tial to  life  while  this  is  not  true  of  carbohydrates  and  fats, 
since  the  proteids  must  be  used  to  build  up  new  cells  to  take 
the  place  of  those  being  constantly  worn  out  and  eliminated. 

The  animal  foods  which  are  richest  in  proteids  are  lean 
meat,  milk,  eggs,  cheese  and  all  kinds  of  fish,  while  the  vege- 
table are  wheat,  beans,  peas  and  oatmeal.  It  has  been  found 
that  the  animal  proteid  foods  are  split  up  and  digeste'd  much 
more  easily  than  are  the  vegetable.  Hence  the  great  ma- 
jority of  the  people  rely  upon  the  animal  foods  for  their  sup- 
ply of  proteid  material  which  is  necessary  to  life. 

The  composition  of  a  few  of  the  more  important  articles 
used  as  food  is  shown  by  the  following  tables.* 

Milk :  Woman,  Cow, 

Per  cent.  Per  cent. 

Protein   (chiefly  caseinogen) 1.7  3.5 

Butter    (fat) '.....  3.4  3.7 

Lactose    6.2  4.9 

Salts    0.2  0.7 

Eggs : 

Total  amount  of  solid 13.3  per  cent 

These  tables  are  taken  from  Halliburton's  Handbook  of  Physi- 
ology. 


68  THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

Protein    12.2  per  cent. 

Sugar 0.5  per  cent. 

Fats    -} 

Lecithin    t  Traces. 

Choiesterin    | 

Inorganic   salts    0.6  per  cent. 

Meats: 

Ox.  Calf.  Pig.  Fowl.  Pike. 

Water    76.7  75.6  72.6  70.8  79.3 

Solids    23.3  24.4  27.4  29.2  20.7 

Proteins    20.0  19.4  19.9  22.7  18.3 

Fats   ...s 1.5  2.9  6.2  4.1  0.7 

Carbohydrates . .  0.6  0.8  0.6  1.3  0.9 

Salts    1.2  1.3  i.i  i.i  0.8 

Vegetable  Foods: 

Wheat.  Barley.    Oats.      Rice.       Peas.  Potatoes. 

Water 13.6  13.8  12.4  13.1  14.8  76.0 

Protein    12.4  n.i  10.4  7.9  23.7  2.0 

Fat    1.4  2.2  5.2  0.9  1.6  0.2 

Starch    67.9  64.9  57.8  76.5  49.3  20.6 

Cellulose    ...     25  5.3  11.2  0.6  7.5  0.7 

Mineral  salts     1.8  2.7  3.0  i.o  3.1  i.o 

DIGESTION. 

Object. — Digestion  is  largely  a  chemical  process.  Certain 
physical  phenomena  are  auxiliary.  The  foods  not  yielding 
energy  are  not  affected  in  a  chemical  way  by  digestion.  They 
are  simply  dissolved,  if  not  already  in  solution,  and  are  dis- 
charged from  the  body  in  the  same  condition  in  which  they 
entered.  But  the  other  classes  of  food  must  either  be  separ- 
ated from  innutritions  substances  with  which  they  enter,  or 
undergo  certain  changes  themselves,  or  both,  before  they 
can  be  absorbed  and  assimilated.  This  necessitates  a  com- 
plicated digestive  apparatus  and  the  subjecting  of  different 
classes  of  food  to  different  digestive  fluids  and  other  gastro- 
intestinal influences.  The  object  of  digestion  is  therefore 
twofold,  first,  to  convert  the  foods  into  soluble  materials  and, 


DIGESTION  69 

second,  to  bring  about  such  changes  in  their  composition  as 
will  insure  their  absorption  and  appropriation  by  the  tissues. 

Enzymes. — The  chemical  changes  taking  place  in  digestion 
are  of  a  peculiar  nature,  in  that  they  are  effected  largely  by 
the  presence  of  substances  known  as  enzymes,  correspond- 
ing in  an  obscure  way  with  ordinary  chemical  reagents. 
These  have  been  called  unorganized  or  unformed  ferments, 
to  distinguish  them  from  such  organized  ferments  as  bac- 
teria, yeast,  fungi,  etc.  They  are  not  themselves  possessed 
of  any  vital  activity,  though  formed  in  living  organisms,  like 
plants  or  animals.  They  are  of  indefinite  chemical  composi- 
tion, contain  nitrogen  and  are  supposed  to  be  of  proteid 
structure.  The  characteristic  point  in  their  action  has  been 
supposed  to  be  that  they  produce  a  chemical  change  without 
themselves  being  affected  by  that  change.  This  is  doubtless 
practically  true,  but  it  is  found  in  experimental  work  that  "a 
given  solution  of  enzyme  cannot  be  used  over  and  over  again 
indefinitely."  It  finally  loses  its  identity. 

According  to  the  foods  on  which  they  act  and  the  effects 
they  produce,  enzymes  are  classified  as :  ( i )  Proteolytic 
enzymes,  which  convert  proteids  into  soluble  peptones ;  ex- 
amples are  pepsin  and  trypsin.  (2)  Amylolytic  enzymes, 
which  convert  starches  into  sugar ;  examples  are  ptyalin  and 
amylopsin.  (3)  Fat-splitting  enzymes,  which  convert  neu- 
tral fats  into  glycerine  and  fatty  acids ;  an  example  is  steap- 
sin.  (4)  Sugar-splitting  enzymes,  which  convert  the  non- 
absorbable  (saccharose)  into  absorbable  (dextrose)  sugar; 
an  example  is  invertase.  (5)  Coagulating  enzymes,  which 
precipitate  soluble  proteids ;  an  example  is  rennin. 

Characteristics  of  Enzymes. — Some  of  the  characteristics 
of  enzymes  are  as  follows :  ( I )  They  are  soluble  in  water 
and  in  glycerine.  (2)  In  solution  they  are  destroyed  before 
the  boiling  point  is  reached  (140°  to  180°  Fahrenheit).  Very 
low  temperatures  do  not  destroy  them,  but  suspend  their  ac- 
tion. (3)  They  never  completely  convert  the  substance  upon 
which  they  act.  It  is  supposed  that  the  substance  produced, 


JO          THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

as  peptones  for  example,  have  an  inhibitory  action  upon  the 
enzyme.  If  these  substances  be  removed  as  they  are  formed, 
the  action  of  the  enzyme  continues.  (4)  The  particular  re- 
sult is  independent  of  the  amount  of  the  enzymes  (unless 
it  be  very  small)  no  matter  how  large  a  quantity  of  the  sub- 
stance to  be  acted  upon  is  present. 

Manner  of  Action. — These  enzymes  are  supposed  to  bring 
about  their  respective  changes  through  hydrolysis — that  is, 
by  causing  water  to  be  taken  up  by  the  molecules  of  the  af- 
fected substance  and  by  the  subsequent  splitting  of  the 
newly  formed  molecule  into  two  or  more  simpler  ones.  How 
they  cause  this  appropriation  of  water  is  as  yet  undeter- 
mined. It  was  formerly  supposed  to  be  brought  about  by 
contact  merely,  and  the  enzymes  were  called  catalytics;  but 
this  term  offers  no  explanation  of  the  real  change  which  oc- 
curs. 

Digestive  Processes. — The  digestive  processes  may  be  con- 
sidered under  the  heads  of  (i)  prehension,  (2)  masti- 
cation, (3)  salivary  digestion,  (4)  deglutition,  (5)  gastric 
digestion,  and  (6)  intestinal  digestion.  Prehension,  mastica- 
tion and  deglutition  cannot  properly  be  looked  upon  as  di- 
gestive processes,  inasmuch  as  they  involve  no  chemical 
change.  They  are,  however,  necessary  occurrences,  and  can- 
not be  disregarded.  Of  course,  absorption  and  "internal  di- 
gestion" follow  gastro-intestinal  or  "external  digestion,"  and 
assimilation  or  cell  appropriation  follows  absorption. 

Prehension. 

Prehension  is  simply  the  taking  of  food  into  the  mouth. 
Its  mechanism  in  the  human  adult  is  so  familiar  that  it  needs 
no  description.  In  the  sucking  child  it  is  more  complex. 
The  buccal  cavity  is  closed  posteriorly  by  the  application  of 
the  velum  palati  to  the  base  of  the  tongue.  The  tip  of  the 
tongue  is  applied  to  the  hard  palate,  and  successive  portions 
of  it  (going  backward)  being  applied  in  the  same  way  leave 


SALIVARY  GLANDS  7 1 

a  partial  vacuum  in  front,  and  liquids  are  drawn  into  the 
mouth.    The  mechanism  of  drinking  is  the  same. 

Digestion  in  the  Mouth. 

Mastication. — The  object  of  mastication  is  to  grind  up  the 
food  so  that  it  may  be  swallowed  more  easily  and  the  various 
digestive  fluids,  particularly  the  saliva  and  gastric  juice,  may 
have  more  ready  access  to  its  parts.  The  proper  mastication 
of  the  food  is  an  important  factor  in  its.  complete  digestion 
later  on. 

Mechanically,  mastication  is  effected  by  the  action  of  the 
lower  jaw,  aided  by  the  tongue,  lips  and  cheeks.  This  re- 
mark presumes  of  course  that  the  teeth  are  intact.  Lateral 
and  antero-posterior  movements  of  the  lower  jaw  combine 
with  its  simple  elevation  to  compress  and  grind  the  food  be- 
tween the  teeth.  The  muscles  which  depress  the  lower  jaw 
are  the  diagastric,  mylohyoid,  geniohyoid  and  platysma. 
Those  which  elevate  it  are'  the  temporal,  masseter,  internal 
and  external  pterygoids.  The  attachments  of  the  external 
pterygoids  are  such  that  by  their  simultaneous  action  the 
mandible  can  be  thrown  forward  and,  by  their  alternate  con- 
traction, from  side  to  side.  The  tongue  is  active  during  mas- 
tication in  carrying  the  mass  of  food  to  this  or  that  part  of 
the  buccal  cavity  so  that  it  may  be  ground  up  completely. 
It  also  gives  accurate  information  as  to  the  size  (of  the  mass) 
and  stage  of  mastication.  The  cheeks,  as  is  shown  in  facial 
palsy,  are  quite  important  in  keeping  the  food  from  between 
them  and  the  teeth.  The  lips  prevent  the  escape  of  liquids 
from  the  mouth,  in  addition,  to  assisting  in  prehension. 

The  Salivary  Glands  and  Their  Secretion. 

The  first  of  the  digestive  juices  with  which  the  food  comes 
in  contact  is  the  saliva  which  is  the  mixed  secretion  of  the 
large  salivary  glands  and  the  various  smaller  mucous  and 


72          THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

serous  glands  which  open  into  the  mouth  cavity.  The  chief 
salivary  glands  are  three  in  number  on  each  side  of  the 
mouth — the  parotid,  submaxillary  and  sublingual.  Besides 
these,  there  are,  throughout  the  buccal  mucous  membrane,  a 
number  of  smaller  glands  of  similar  structure  contributing 
to  the  formation  of  saliva.  The  parotid  gland  is  situated 
just  beneath  and  in  front  of  the  lobe  of  the  ear;  the  sub- 
maxillary beneath  the  mandible  about  the  center  of  the  base 
of  the  submaxillary  triangle,  and  the  sublingual  beneath  the 
mucous  membrane  of  the  mouth,  just  lateral  to  the  lingual 
frenum. 


FIG-  33. — Cells  of  the  alveoli  of  a  serous  or  watery  salivary  gland. 
(Brubaker  after   Yeo.} 

A,  after  rest;  B,  after  a  short  period  of  activity;  C,  after  a  prolonged  period 
of  activity. 

The  duct  from  the  parotid,  Stenson's  duct,  runs  beneath 
the  mucous  membrane  of  the  cheek  to  a  point  opposite  the 
second  upper  molar  tooth,  where  is  its  opening  into  the 
mouth.  The  duct  from  the  submaxillary,  Wharton's  duct, 
discharges  the  secretion  from  that  gland  into  the  mouth  by 
the  side  of  the  frenum  of  the  tongue.  The  secretion  from 
the  sublingual  reaches  the  mouth  by  a. number  of  small  ducts 
(Rivinus)  which  open  also  by  the  side  of  the  frenum,  and 
sometimes  as  well  by  a  larger  duct,  Bartholin's,  which  runs 
parallel  with  Wharton's  and  empties  near  it. 

Histology. — In  structure  the  salivary  glands  have  been 
shown  to  be  of  the  compound  tubular  variety,  the  secreting 
part  being  tubular.  The  parotid  is  a  serous  gland,  the  other 


SALIVARY  GLANDS 


73 


two  are  usually  said  to  be  mucous,  though  they  contain  both 
serous  and  mucous  cells.  The  ducts  subdivide  into  smaller 
ducts  and  tubes,  until  a  distinct  tubule  is  distributed  to  every 
acinus  and  becomes  the  lumen  of  that  acinus.  The  whole 
arrangement  resembles  the  branchings  of  a  tree. 

The  flow  from  these  glands  is  greatly  increased  by  masti- 
cation. From  the  parotid  the  flow  is  much  more  abundant 
on  that  side  upon  which  the  mastication  takes  place.  During 
activity  it  can  be  shown  that  the  granules  of  the  serous  cells 
accumulate  toward  the  lumen  of  the  acinus  while  the  outer 


FIG.  34. — Section  of  a  mucous  gland.  (Brubaker  after  Lavdowsky.} 

A,  in  a  state  of  rest;  B,  after  it  has  been  for  some  time  actively  secreting. 

segment  of  the  cells  becomes  comparatively  clear.  It  is  sup- 
posed that  this  is  an  essential  step  in  the  production  of  the 
organic  constituents  of  the  secretion — that  the  granules  con- 
tain either  the  ptyalin  or  the  substance  necessary  to  its  for- 
mation. It  is  also  supposed  that  at  the  same  time  that  the 
ptyalin  is  being  thus  produced  and  discharged,  very  active 
constructive  changes  are  occurring  in  the  clear  zone  of  the 
cells.  During  activity  some  at  least  of  the  mucous  cells  seem 
to  break  down,  but  it  is  probable  that  the  granules  in  the  cell 
protoplasm  become  converted  into  mucin,  which,  being  ex- 
truded, seem  to  destroy  the  cell  itself. 


74  THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

Composition  and  Properties  of  Saliva. — While  it  is  possi- 
ble to  draw  certain  distinctions  between  the  saliva  from  the 
different  glands,  these  distinctions  are  comparatively  unim- 
portant, so  far  as  digestion  is  concerned;  for  the  secretions 
from  the  three  pairs  of  glands  become  mixed  in  the  mouth, 
and  it  is  their  combined  effect  which,  in  any  particular  case, 
is  observed.  Saliva  contains  in  1,000  parts  about  994  of 
water,  the  remaining  six  parts  being  organic  and  inorganic 
solids. 

These  solids  are  chiefly  mucin,  ptyalin,  albumin  and  salts. 
The  salts  are  mainly  the  chlorides  of  sodium  and  potassium, 
the  sulphates  of  potassium,  the  phosphates  of  potassium,  so- 
dium, calcium  and  magnesium,  and  sulphocyanide  of  potas- 
sium. The  mucin  gives  the  ropy  consistence  to  the  fluid  and 
serves  a  mechanical  purpose  only.  The  sulphocyanide  of  po- 
tassium is  unusual  in  the  body  secretions  and  its  presence 
here  is  interesting.  It  may  represent  an  end  product  of  pro- 
teid  metabolism.  The  true  digestive  value  of  saliva  is  due  to 
ptyalin,  an  amylolytic  enzeme. 

Were  it  not  for  the  presence  of  epithelial  cells  in  suspen- 
sion, saliva  would  be  clear  and  transparent.-  Its  reaction  is 
alkaline,  its  specific  gravity  is  about  1004  to  1008,  and  the 
average  amount  of  daily  secretion  is  about  2^4  pounds. 

The  parotid  saliva  is  much  more  watery  and  mixes  much 
more  readily  with  the  food  than  the  submaxillary  and  sub- 
lingual,  which  latter  is  mucilaginous  and  gives  to  the  bolus  a 
glairy  coating.  The  sublingual  saliva  is  thicker  and  more 
viscid  than  the  submaxillary. 

Nerve  Supply. — The  connection  of  the  nervous  system 
with  salivary  secretion  deserves  particular  attention,  since  the 
phenomena  presented  under  its  influence  are  typical,  and, 
if  not  explanatory  of  occurrences  elsewhere  in  the  body,  are 
at  least  very  suggestive. 

Each  one  of  the  three  glands  is  supplied  with  both  cere- 
bro-spinal  and  sympathetic  fibers.  Each  one  of  them  has 
three  kinds  of  nerve  fibers,  secretory,  vaso-dilator  and  vaso- 


SALIVARY  GLANDS  75 

constrictor.  The  secretory  and  vaso-dilator  reach  the  gland 
in  the  cerebro-spinal  trunks ;  the  vaso-constrictor  in  the  sym- 
pathetic. The  vaso-constrictors  and  vaso-dilators  are  dis- 
tributed to  the  walls  of  the  blood  vessels,  and  influence  secre- 
tion indirectly  only  by  increasing  or  diminishing  the  amount 
of  blood  going  to  the  glands.  The  secretory  fibers  exert  their 
influence  directly  upon  the  gland  cells.  It  is  claimed  also 
that  the  secretory  fibers  are  divided  into  sets  controlling  the 
production  of  the  energy-yielding  constituents  and  sets  con- 
trolling the  production  of  water  and  salts. 

The  parotid  gland  receives  its  cerebro-spinal  fibers  through 
a  branch  of  the  fifth  nerve,  but  when  they  are  traced  back- 
ward it  can  be  shown  that  they  are  in  the  tympanic  branch 
of  the  ninth,  and  pass  from  this  branch  to  the  small  super- 
ficial petrosal  nerve  and  thence  to  the  optic  ganglion — from 
which  ganglion  they  run  to  the  parotid  gland  by  the  way  of 
the  auricula-temporal  branch  of  the  third  division  of  the 
fifth.  The  cervical  sympathetic  also  sends  fibers  to  this  gland. 

The  submaxillary  and  sublingual  glands  are  supplied  by 
the  same  nerves.  Their  cerebro-spinal  fibers  leave  the  brain 
by  way  of  the  facial,  follow  the  chorda  tympani  as  far  as  a 
short  distance  beyond  its  junction  with  the  lingual  nerve, 
and  then  leave  it  to  reach  the  submaxillary  ganglion  and 
run  thence  to  the  submaxillary  and  sublingual  glands.  These 
glands  receive  sympathetic  fibers  from  the  superior  cervical 
ganglion. 

Influence  of  Nerve  Supply. — Taking  the  parotid  as  an  ex- 
ample, it  is  found  that  stimulation  of  its  cerebro-spinal  fibers 
produces  an  abundant  watery  flow  of  saliva ;  the  gland  be- 
comes decidedly  redder,  pulsation  is  sometimes  apparent, 
and  it  is  evident  that  the  amount  of  blood  is  locally  increased. 
When  the  sympathetic  supply  of  the  parotid  is  stimulated,  the 
secretion  is  inhibited  or  reduced  to  a  minimum,  the  gland  be- 
comes pale  and  the  amount  of  blood  in  it  is  evidently  dimin- 
ished. 

Similar  corresponding  results  are  occasioned  in  the  sub- 


/6  THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

maxillary  and  sublingual  glands  by  stimulation  of  the  chorda 
tympani  and  the  sympathetic  fibers. 

It  would  seem  at  first,  in  the  light  of  the  vascular  changes 
accompanying  stimulation  of  the  two  supplies  to  all  these 
glands,  that  the  resultant  phenomena  could  be  explained  en- 
tirely by  variations  in  the  amount  of  blood,  and  that  the  ner- 
vous system  influences  their  secretion  only  by  contraction  and 
dilatation  of  the  vessels.  However,  a  number  of  circum- 
stances, which  it  is  unnecessary  to  relate  here,  prove  that  the 
secretory  fibers  exert  an  influence  directly  upon  the  cells 
themselves,  causing  them  to  secrete.  The  mere  distribution 
of  these  fibers  to  the  gland  cells  presupposes  some  such  func- 
tion on  their  part ;  and  it  can  actually  be  shown  that  the  se- 
cretion can  be  increased  when  the  blood  supply  is  cut  off,  or 
without  dilatation  of  the  vessels.  Such  action,  however,  is 
of  course  only  temporary,  for  the  materials  for  secretion 
must  be  supplied  by  the  blood.  The  exact  method  of  ter- 
mination of  the  secretory  fibers  has  not  been  determined. 
It  is  probable  that  they  end  between  and  around  the  cells  and 
do  not  penetrate  their  substance. 

Section  of  the  chorda  tympani  causes  a  continuous  flow 
of  saliva  from  the  submaxillary  and  sublingual  glands  for 
several  weeks.  This  has  been  termed  paralytic  secretion, 
and  is  supposed  to  be  due  to  the  fact  that  the  chorda  fibers 
do  not  themselves  run  directly  to  the  glands,  but  are  distrib- 
uted to  sympathetic  ganglia  (the  submaxillary  or  others  in 
the  gland  substance).  Section  of  the  chorda,  then,  causes 
degeneration  of  its  fibers  only  as  far  as  these  ganglia,  and 
their  cells  are  thought  to  be  subject,  in  some  obscure  way,  to 
continuous  irritation  (luring  the  period  for  which  the  para- 
lytic secretion  continues. 

Function. — The  function  of  this  secretion  is  twofold,  (a) 
mechanical,  and  (b)  chemical. 

(a)  From  a  mechanical  standpoint  (i)  it  facilitates  pho- 
nation,  mastication  and  gustation  by  maintaining  a  proper 
degree  of  moisture  in  the  mouth;  (2)  its  more  watery  parts 


SALIVARY  GLANDS  77 

(parotid)  mix  with  the  food,  dissolving  part  of  it,  so  that  it 
may  be  more  easily  masticated  and  swallowed  while  its  more 
viscid  parts  ( submaxillary  and  sublingual)  spread  over  the 
surface  of  the  bolus  to  aid  in  deglutition. 

(b)  From  a  chemical  standpoint,  the  function  of  the  saliva 
is  to  convert  starch  into  sugar.  It  does  this  through  the 
agency  of  its  enzyme,  ptyalin,  which  conforms  to  the  char- 
acteristics of  enzymes  already  noted.  Maltose  (Ci2H22Oii 
H-HaO)  is  the  form  of  sugar  produced,  but  there  are  several 
intermediate  substances  formed  before  maltose  finally  re- 
sults. The  starch  molecule  (CeHioOs)  was  formerly  sup- 
posed to  simply  appropriate  a  molecule  of  water  to  form 
dextrose  (grape  sugar,  glucose,  CeH^Oe),  but  it  is  now 
thought  that  there  is  a  succession  of  hydrolytic  changes  with 
the  production  of  dextrin  and  maltose.  That  is,  the  starch 
molecule  appropriates  a  molecule  of  water;  this  new  mole- 
cule splits  into  a  certain  kind  of  dextrin  and  maltose*  the 
dextrin  left  itself  appropriates  water  and  splits  up  into  an- 
other kind  of  dextrin  and  maltose;  this  last  dextrin  goes 
through  a  similar  process  with  a  like  result,  until  finally  only 
maltose  is  produced.  Some  dextrose  may  be  produced.  It 
will  be  seen  under  gastric  digestion  that  mineral  acids  will 
also  convert  starch  into  sugar,  but  in  this  case  the  form  of 
sugar  is  dextrose. 

The  effect  of  temperature  on  the  action  of  enzymes  has 
been  noticed.  The  optimum  for  ptyalin  is  100°  Fahrenheit 
The  reaction  of  saliva  is  alkaline  and  its  effect  on  starch  is 
stopped  by  an  acid  medium,  since  the  enzyme  is  thereby  de- 
stroyed. However,  ptyalin  has  been  shown  to  act  even  a 
little  better  in  perfectly  neutral  than  in  alkaline  solutions 
(Chittenden).  The  action  of  this  substance  on  starch  is  very 
much  facilitated  if  the  starch  be  cooked;  in  fact,  its  action 
on  uncooked  starch  is  so  slow  that  probably  it  is  inconse- 
quential in  digestion.  Cooked  starch  becomes  hydrated,  and 
furthermore  has  its  cellulose  capsule  removed  from  the 
granulose,  both  of  which  circumstances  make  if  much  more 
susceptible  to  salivary  influences. 


78  THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

However,  it  must  be  admitted  that  the  practical  effect  of 
ptyalin  in  digestion  is  not  very  considerable  in  the  mouth 
mainly  because  the  food  is  not  kept  in  the  mouth  long 
enough.  However,  large  quantities  of  saliva  are  swallowed 
with  the  food  and  it  continues  its  action  in  the  stomach 
while  the  food  is  stored  in  the  cardiac  end  and  only  ceases 
its  activity  when  the  food  is  thoroughly  mixed  with  the  acid 
gastric  juice.  The  conversion  of  starch  into  sugar  is  con- 
tinued and  concluded  in  the  small  intestine. 

Deglutition. 

The  act  of  deglutition  is  commonly  divided  into  three  pe- 
riods, depending  upon  the  part  through  which  the  food  is 
passing.  During  the  first  period  the  bolus  passes  from  the 
mouth  through  the  isthmus  of  the  fauces,  during  the  second 
through  the  pharynx,  and  during  the  third  through  the 
esophagus  into  the  stomach.  A  brief  reference  to  the  anat- 
omy of  these  parts  is  necessary. 

Fauces. — The  isthmus  of  the  fauces  is  the  opening  at  the 
back  of  the  mouth,  bounded  below  by  the  base  of  the  tongue, 
and  above  by  the  soft  palate  and  uvula,  and  laterally  by  the 
pillars  of  the  fauces,  between  which  are  the  tonsils.  The 
anterior  pillars  are  easily  seen  when  the  mouth  is  opened 
widely,  and  consist  of  the  palatoglossi  muscles  with  their 
covering  mucous  membrane.  The  posterior  pillars  approach 
each  other  more  nearly  than  the  anterior,  and  consist 
of  the  palatopharyngei  muscles  and  their  covering  mucous 
membrane. 

Pharynx. — The  pharynx  extends  from  the  basilar  process 
of  the  occipital  bone  above  about  four  and  a  half  inches 
downward.  It  communicates  with  the  posterior  nares,  the 
mouth,  the  Eustachian  tubes,  the  larynx  and  esophagus.  The 
tube  is  made  up  of  two  coats,  an  external  muscular  and  an 
internal  mucous.  The  muscular  coat  consists  of  the  three  con- 
strictors and  the  stylopharyngeus.  The  mucous  coat  is  cov- 


DEGLUTITION  79 

ered  in  its  upper  part  with  columnar  ciliated  and  its  lower 
part  by  pavement  epithelium. 

Esophagus. — The  esophagus  runs  a  course  of  about  nine 
inches  from  the  end  of  the  pharynx,  at  a  point  behind  the 
cricoid  cartilage,  to  the  stomach,  which  it  enters  a  little  to  the 
left  of  the  median  line.  The  coats  of  the  esophagus  are  two, 
an  external  muscular  and  an  internal  mucous.  The  external 
coat  has  its  fibers  disposed  in  two  layers,  longitudinal 
and  circular.  The  circular  layer  is  internal.  In  the  upper 
third  of  the  esophagus  the  fibers  of  the  muscular  coat  are  all 
striped,  but  at  the  beginning  of  the  middle  third  they  begin  to 
give  place  to  plain  fibers,  and  these  latter  progressively  in- 
crease, to  constitute  virtually  the  whole  muscular  coat  at  the 
diaphragm.  The  internal  mucous  coat  is  lined  by  squamous 
epithelium.  This  is  thrown  into  longitudinal  folds  except 
during  the  passage  of  substances  through  the  esophagus. 
The  outside  fibrous  tissue  attaches  the  whole  esophagus  to 
the  surrounding  tissue. 

Mechanism  of  Deglutition. — The  first  period  of  degluti- 
tion is  voluntary  but  automatic,  like  respiration.  The  mor- 
sel of  food  is  forced  toward  and  through  the  fauces  by  the 
tongue,  which  presses  from  before  backward  against  the 
hard  palate,  with  the  bolus  above  it.  That  the  tongue  is 
mainly  concerned  in  this  act  is  shown  by  inability  to  swallow 
when  this  organ  is  absent,  unless  the  food  is  pushed  far  back 
into  the  mouth  by  the  finger  or  other  means. 

The  mechanism  of  the  second  period  is  much  more  com- 
plex. The  food  must  pass  through  the  pharynx  into  the 
esophagus,  and  must  not  be  allowed  to  enter  any  of  the  other 
openings  communicating  with  the  pharynx.  The  larynx  es- 
pecially is  to  be  protected.  Since  the  air  enters  through  the 
posterior  nares  above  the  isthmus  and  must  enter  through 
the  larynx  in  front  of  the  esophagus,  it  follows  that  the  cur- 
rent of  air  would  cross  the  current  of  food  if  swallowing 
and  respiration  took  place  together.  Consequently  respira- 
tion is  suspended  during  deglutition.  A.S  soon  as  the  food 


8O          THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

has  passed  the  fauces,  the  elevators  of  the  hyoid  raise  that 
bone,  and  with  it  the  larynx.  It  is  at  the  same  time  pulled 
a  little  forward,  and  since  the  pharynx  is  attached  to  the 
larynx  posteriorly,  the  former  necessarily  follows  the  move- 
ment of  the  latter,  and  is  thus  slipped  under  the  base  of  the 
tongue  and  the  entering  bolus.  With  elevation  of  the  larynx 
the  superior  constrictor  of  the  pharynx  contracts  upon  the 
food,  and  passes  it  quickly  to  the  grasp  of  the  middle  con- 
strictor, which  in  turn  hands  it  to  the  inferior  constrictor  and 
thence  to  the  esophagus. 

The  posterior  nares  are  protected  by  contraction  of  the 
posterior  pillars  and  the  superior  constrictor.  The  laryngeal 
opening  is  protected  by  the  epiglottis.  When  the  tongue  is 
forced  back  and  the  larynx  raised  the  natural  effect  would 
be  to  fold  the  epiglottis  down  over  the  laryngeal  opening. 
At  the  same  time  contraction  of  the  pharyngeal  muscles 
draws  together  the  sides  of  the  larynx  and  aids  in  closing 
the  glottis.  Furthermore,  the  vocal  cords  fall  together  (as 
they  always  lie  except  during  inspiration — and  inspiration  is 
now  suspended). 

The  third  period  passes  the  food  through  the  esophagus 
into  the  stomach  by  contraction  from  above  downward  of 
successive  portions  of  its  muscular  wall.  Contraction  of  the 
longitudinal  fibers  draws  the  mucous  membrane  above  the 
bolus.  Then  the  circular  fibers,  contracting  in  successive 
segments  from  above  downward,  force  the  bolus  before 
them.  These  movements  are  continued  until  the  food 
reaches  the  stomach.  The  time  consumed  in  swallowing  a 
given  article  is  about  six  seconds. 

This  is  the  mechanism  which  carries  all  materials  through 
the  alimentary  canal  from  the  esophagus  to  the  anus.  It  is 
called  peristalsis,  or  vermicular  (worm-like)  action. 

Nervous  Control. — While  nearly  all  the  muscular  tissue 
concerned  in  deglutition  is  of  the  striated  variety,  the  whole 
process,  except  the  first,  which  is  automatic,  must  be  consid- 
ered as  reflex.  The  mechanism  of  deglutition  is  one  of  the 


DIGESTION  AND  ABSORPTION  IN  THE  STOMACH  8l 

best  examples  of  finely  coordinated  muscular  action  to  be 
found.  The  afferent  fibers  concerned  are  from  the  5th,  gth, 
and  loth,  and  the  superior  laryngeal  branch  of  the  last.  The 
efferent  fibers  are  from  the  5th,  7th,  gth,  loth,  and  I2th. 
The  center  for  the  reflex  is  supposed  to  be  far  forward  in 
the  medulla. 

It  ought  to  be  added  that  the  Kronecker-Meltzer  theory 
of  deglutition  assails  with  considerable  plausibility  the  me- 
chanism of  deglutition  as  above  given.  In  a  word,  this  the- 
ory holds  that  when  the  bolus  of  food  rests  upon  the  dor- 
sum  of  the  tongue,  and  the  tip  of  that  organ  prevents,  by  its 
apposition  to  the  hard  palate,  the  escape  of  the  food  forward, 
the  mylohyoids  contract  with  great  force,  compress  the  food, 
and  it  escapes  by  the  route  of  least  resistance,  which  is  back- 
ward. It  is  thus  shot  into  the  esophagus,  and  the  contraction 
of  the  pharyngeal  muscles  only  supplements  that  of  the  my- 
lohyoids. 

Digestion  and  Absorption  in  the  Stomach. 

Anatomy. — The  istomach  is  situated  beneath  the  dia- 
phragm in  the  upper  part  of  the  abdominal  cavity,  and  is 
moored  by  the  esophagus  and  folds  of  the  peritoneum.  Its 
general  shape  has  been  compared  to  that  of  the  bagpipe.  Its 
large,  or  fundic,  end  is  to  the  left;  its  small,  or  pyloric,  to 
the  right.  By  far  the  greater  part  of  the  organ  is  to  the  left 
of  the  median  line.  A  very  considerable  portion  is  to  the 
left  of  the  esophageal  opening.  Except  when  distended,  its 
anterior  and  posterior  walls  hang  in  an  approximately  ver- 
tical direction,  and  are  usually  in  contact  by  their  mucous 
surfaces.  Its  greatest  length  when  moderately  distended  is 
about  fourteen  inches,  its  transverse  diameter  about  five 
inches,  and  its  capacity  about  five  pints.  At  the  point  where 
the  anterior  and  posterior  walls  meet  inferiorly,  the  great 
omentum  (the  peritoneum  from  the  two  walls)  is  given  off. 
This  is  the  greater  curvature  and  has  the  gastro-epiploica 

6 


82  THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 


FIG.  35. — Human  alimentary  canal. 

a,  esophagus;  b,  stomach;  c,  cardiac  orifice;  d,  pylorus;  e,  small  intestine;  f, 
biliary  duct;  g,  pancreatic  duct;  h,  ascending  colon;  i,  transverse  colon;  ;',  de- 
scending colon;  k,  rectum.  (Collins  &  Rockwell.) 


DIGESTION  AND  ABSORPTION   IN   THE  STOMACH  83 

dextra  and  the  gastro-epiploica-sinistra  arteries  running 
along  it  between  the  two  folds  of  the  omentum.  Where  the 
anterior  and  posterior  walls  meet  superiorly,  the  stomach  is 
joined  by  the  lesser  omentum,  the  two  layers  of  which  are 
continued  in  front  and  behind  as  the  serous  covering  of  the 
stomach.  This  is  the  lesser  curvature,  and  has  the  gastric 
and  pyloric  branch  of  the  hepatic  arteries  running  along  be- 
tween the  two  layers  of  the  lesser  omentum.  The  large  left 
hand  portion  of  the  stomach  cavity  'is  called  the  fundus  or 
greater  pouch.  The  opposite  portion  of  the  cavity  is  called 
the  lesser  pouch  or  antrum  pylori.  At  one  end  is  the 
cardiac  or  esophageal  opening,  at  the  other  the  pyloric. 

Histology. — The  coats  of  the  stomach  walls  are  four. 
From  without  inward  these  are  the  (i)  peritoneal,  or  serous, 
(2)  muscular,  (3)  submucous,  and  (4)  mucous. 

1.  The  peritoneal  coat  covers  the  whole  of  the  organ  ex- 
cepting an  inconsiderable  linear  area,  where  the  two  layers 
of  the  lesser   (gastro-hepatic)   omentum  join  it  along  the 
lesser  curvature,  and  a  similar  area  along  the  greater  curva- 
ture, where  the  serous  coats  of  the  anterior  and  posterior 
walls  leave  the  organ  to  form  the  great  omentum.    This  coat 
is  simply  a  fold  given  off  from  the  peritoneum  to  envelop  the 
stomach  in  practically  the  same  manner  as  the  other  abdomi- 
nal viscera.     Its  structure  is  that  of  serous  membranes  in 
general. 

2.  The  muscular  coat,  varying  in  thickness  from  %o  in. 
over  the  fundus  to  M.2  in.  at  the  pylorus,  is  disposed  in  three 
layers,   (a)   the  external  longitudinal,   (b)   middle  circular, 
and  (c)  internal  oblique.    The  longitudinal   fibers  are  con- 
tinued from  the  corresponding  fibers  of  the  esophagus.  They 
are  marked  along  the  lesser  curvature,  but  not  very  distinct 
over  other  parts.    The  circular  fibers  are  not  abundant  to  the 
left  of  the  esophageal  opening.    They  progressively  increase 
toward  the  right,  and  at  the  pyloric  opening  constitute  a  dis- 
tinct  and   powerful   muscular   ring,   the   pyloric  sphincter, 
which,  projecting  into  the  lumen  presents  a  more  or  less 


84          THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

flat  surface  on  the  duodenal  side  to  prevent  the  regurgita- 
tion  of  food.  The  oblique  fibers  are  supposed  to  be  contin- 
uous with  the  circular  fibers  of  the  esophagus.  They  extend 
over  the  greater  pouch  from  a  point  just  to  the  left  of  the 
esophageal  opening  to  a  point  on  the  greater  curvature,  about 
the  junction  of  the  middle  and  pyloric  thirds.  Here,  at  the 


Serosa.     _: 

FIG.  36.— V.  S.  Wall  of  human  stomach. 
E,  epithelium;  G,  glans;  Mm,  muscularis  mucosae.         15.     (Stirling.) 

fight  hand  limit  of  the  oblique  fibers,  the  stomach  is  said 
during  digestion  to  be  considerably  constricted,  so  that  a  tem- 
porary sphincter  is  established.  This  is  the  point  of  separa- 
tion between  the  fundus  and  the  antrum  pylori,  and  is  some- 
times called  the  sphincter  antri  pylorici.  The  fibers  of  the 
muscular  coat  are  of  the  plain  variety,  as  is  all  the  gastro-in- 


GASTRIC  GLANDS  85 

testinal  muscular  tissue  from  the  lower  end  of  the  esophagus 
to  the  external  sphincter. 

3.  The  submucous  coat  consists  of  loose  fibro-elastic  con- 
nective tissue  which  allows  free  movement  between  the  mus- 
cular and  mucous  coats.    It  contains  rather  large  blood-ves- 
sels and  a  nerve-plexus,  the  plexus  of  Meissner. 

4.  The  mucous  coat  has  an  average  thickness  of  about  ^5 
in.,  is  loosely  attached  to  the  submucous  coat,  and,  except 
during  gastric  digestion,  is  thrown  into  longitudinal  rugae. 
It  consists  of  columnar  epithelium  resting  upon  a  basement 
membrane,  beyond  (underneath)  which  is  the  capillary  blood 
supply.     Throughout  the  greater  part  of  the  stomach  the 
mucous  membrane  can  be  shown  to  be  divided  by  delicate 
connective  tissue  into  numerous  polygonal  depressions,  from 
the  bottom  of  which  extend  the  gastric  glands. 

The  Gastric  Glands. 

In  the  mucous  membrane  of  the  stomach  are  found  two 
kinds  of  glands.  According  to  their  relative  position  with 
reference  to  the  two  ends  of  the  stomach  they  are  called 
fundic  and  pyloric.  It  is  to  be  noted,  however,  that  neither 
of  these  divisions  is  confined  strictly  to  that  portion  of  the 
stomach  which  its  name  would  seem  to  indicate.  Accord- 
ing to  their  secretion  the  glands  are  called  acid  and  peptic. 
The  fundic  and  acid,  and  the  pyloric  and  peptic  are  consid- 
ered to  be  identical.  But  attention  is  called  to  the  fact  that 
while  peptic  (pyloric)  glands  secrete  pepsin  only,  the  acid 
(fundic)  secrete  both  acid  and  pepsin. 

Structure. — Some  of  the  gastric  glands  are  simple  tubules, 
while  others  may  be  bifurcated,  so  that  two  (or  more)  tu- 
bules communicate  with  the  surface  by  a  single  canal.  They 
may  all,  however,  be  classified  as  belonging  to  the  simple 
tubular  variety.  They  have  a  deep  secreting  portion  and  a 
superficial  non-secreting  portion.  The  latter  is  lined  by 
columnar  epithelium,  and  is  the  duct  proper.  The  former  is 


86 


THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 


lined  by  cuboidal  epithelium  which  discharges  its  secretion 
into  the  lumen,  this  lumen  being  only  a  continuation  of  the 
duct.  These  cuboidal  cells  are  called  peptic  cells  because 


FIG.  37. — Vertical  section  of  the  gastric  mucous  membrane. 

S,  g,  pits  on  the  surface;  p,  neck  of  a  fundus-gland  opening  into  a  duct,  g;  x, 
parietal,  and  y,  chief  cells;  a,  v,  c,  artery,  vein,  capillaries;  d,  d,  lymphatics, 
emptying  into  a  large  trunk,  e.  (Landois.) 

they  produce  pepsin,  or  its  forerunner,  pepsinogen.  The 
fundic  (acid)  glands  are  found  to  have  lying  close  to  the 
basement  membrane  a  number  of  large  cells  at  intervals  be- 


GASTRIC  GLANDS  87 

tween  the  cuboidal  cells  and  not  extending  outward  to  the 
central  lumen.  They  are  thought  to  communicate  with  the 
lumen  by  capillary  ducts,  which  may  even  penetrate  their 
substance.  They  are  supposed  to  secrete  hydrochloric  acid, 


FIG.  38. — Section  of  the  pyloric  mucous  membrane.   (Landois.) 

and  are  called  acid  cells  from  this  fact,  or  parietal  cells 
from  their  position.     (See  Fig.  37.) 
Method   of   Secretion,— When   food   is   ingested   gastric 


88  THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

movements  very  soon  begin,  carrying  the  food  in  this  direc- 
tion or  that,  as  described  later.  At  the  same  time,  the  gastric 
mucous  membrane  changes  from  a  pale  pink  to  a  congested 
red,  and  soon  drops  of  gastric  juice  begin  to  appear.  They 
run  to  the  dependent  portions  of  the  cavity  and  become  in- 
corporated with  the  alimentary  mass.  It  is  believed  that  if 
the  gastric  movements  did  not  occur,  this  secretion  would  be 
limited  for  fifteen  or  thirty  minutes  to  a  very  small  area, 
namely,  that  with  which  the  food  is  in  contact.  But  it  is 
comparatively  general  because  the  movements  bring  practic- 
ally all  parts,  at  least  of  the  fundic  mucous  membrane,  in 
contact  with  the  food  before  this  time  has  elapsed.  The  idea 
is  that  up  to  fifteen  or  thirty  minutes  after  the  introduction 
of  food,  the  glands  are  made  to  secrete  by  direct  mechanical 
stimulation  of  the  food,  and  after  this  time  the  secretion 
becomes  general,  whether  mechanical  irritation  becomes  gen- 
eral or  not. 

It  ought  to  be  added,  however,  that  in  recent  years  secre- 
tion by  mechanical  stimulation  has  been  denied,  and  the  de- 
nial is  supported  by  good  evidence.  Besides  direct  proof  by 
experiments,  it  is  shown  that  this  early  secretion  occurs 
without  mechanical  irritation,  as  when  food  is  chewed  and 
made  to  pass  through  an  esophageal  fistula,  or  even  by  the 
sight  of  food.  These  observers  (Pawlow)  state  that  food 
introduced  into  the  stomach  through  a  fistula  produces  abso- 
lutely no  flow  if  the  animal  experimented  upon  does  not 
know  of  the  introduction.  Under  this  view  the  secretion  is 
a  distinct  reflex,  the  impressions  being  carried  to  the  center 
by  afferent  nerves  distributed  to  the  mouth,  or  by  nerves  o/ 
special  sense. 

Whether  as  a  reflex  or  as  a  result  of  mechanical  stimula- 
tion, the  fact  remains  undisputed  that  the  flow  begins  a  few 
minutes  after  the  introduction  of  food,  and  lasts  until  gas- 
tric digestion  is  completed.  After  a  time  it  is  supposed  that 
chemical  changes  in  the  food  itself  further  stimulate  the  gas- 
tric glands,  through  their  influence  on  the  secretory  nerves. 


GASTRIC  GLANDS  89 

These  stimulating  chemical  products  are  not  developed  alike 
from  all  foods;  and  the  conclusion  is  warranted  that  some 
substances  do  not  undergo  gastric  digestion  so  readily  as 
others.  Ordinary  bread  and  the  whites  of  eggs,  for  example, 
are  said  not  to  develop  them.  It  has  been  further  shown  that 
fats,  oils,  etc.,  actually  develop  substances  which  chemically 
inhibit  gastric  secretion.  There  appears  also  to  be  a  kind  of 
chemical  regulation  of  the  amount  and  quality  of  juice,  ac- 
cording as  much  or  little,  or  varying  acidity,  is  needed  in  the 
digestion  of  the  substance  in  the  stomach. 

Conditions  influencing  digestion  operate  mainly  by  produc- 
ing changes  in  the  quantity  or  quality  of  gastric  juice,  and 
these  changes  in  turn  are  largely  effected  through  the  ner- 
vous system.  Fever,  overeating,  depressing  emotions,  stren- 
uous physical  or  mental  exercise,  etc.,  decrease  the  secretion 
and  correspondingly  interfere  with  digestion. 

Changes  During  Activity. — Like  the  salivary  cells,  the  cu- 
boidal  peptic  cells  can  be  shown  to  undergo  changes  during 
secretory  activity.  When  at  rest  they  contain  abundant  gran- 
ules, but  during  secretion  these  granules  disappear,  first  from 
the  base  and  later  from  well-nigh  the  whole  cell.  The  gran- 
ules are  supposed  to  contain  pepsin,  or  rather  pepsinogen, 
for  it  is  thought  that  pepsin  is  not-formed  by  the  cell  directly, 
but  is  made  out  of  pepsinogen,  which  is  the  product  of  the 
peptic  cells,  probably  under  the  action  of  hydrochloric  acid. 
The  rennin  is  also  supposed  to  exist  in  the  cells  as  some  pre- 
liminary material  corresponding  to  pepsinogen.  This  ma- 
terial may  be  termed  rennin  zymogen. 

Changes  in  the  acid  cells  during  activity  also  occur,  but 
are  more  obscure  than  those  in  the  peptic  cells.  The  source 
of  hydrochloric  acid  is  a  decomposition  of  the  neutral  chlor- 
ides of  the  blood  and  the  union  of  the  chlorine  thus  liber- 
ated with  hydrogen,  but  how  or  why  this  occurs  is  not  ex- 
plained by  phenomena  so  far  observed. 

Secretory  Nerves. — While  it  has  been  impossible  to  de- 
monstrate secretory  fibers  to  the  cells  of  the  gastric  glands, 


9O  THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

such  fibers'  must  exist  in  the  vagifc.  Section  of  it  (and  the 
sympathetic),  however,  does  not  entirely  stop  the  secretion, 
but  incidents  referred  to  in  a  preceding  section,  such  as  se- 
cretion at  sight  of  food,  or  when  food  is  chewed  and  not 
swallowed,  certainly  point  to  an  influence  of  the  central  sys- 
tem over  secretion.  Of  course  the  sympathetic  fibers  to  the 
vessel  walls  are  indirectly  concerned. 

Condition  of  Food  on  Entering  Stomach. — The  food  en- 
ters the  stomach  in  the  same  condition  in  which  it  left  the 
mouth.  It  has  been  more  or  less  completely  triturated  by 
mastication ;  the  whole  has  been  moistened,  and  a  part  dis- 
solved by  the  saliva.  All  the  materials  taken  in  have  been 
thoroughly  mixed  with  each  other,  and  some  of  the  starch 
has  been  converted  into  sugar.  The  reaction  is  now  alka- 
line, unless  the  acidity  of  the  articles  taken  has  been  too 
great  to  be  overcome  by  the  alkalinity  of  the  saliva — in  which 
case  there  would  be  no  amylolytic  change.  Excepting  starch, 
all  foods  entering  the  stomach  are  chemically  unaffected.  It 
remains  to  see  what  happens  to  the  foods  under  the  influence 
of  gastric  digestion.  These  changes  are  brought  about  by 
the  gastric  juice  aided  by  muscular  movements  of  the  stom- 
ach. 

Properties  and  Composition  of  Gastric  Juice. — The  secre- 
tion of  the  glands  of  the  stomach  is  called  gastric  juice.  Gas- 
tric juice  may  be  secured  in  several  ways,  but  the  most  reli- 
able article  for  experimentation  is  taken  from  a  previously 
established  gastric  fistula  in  one  of  the  lower  animals.  It  is 
a  thin,  almost  colorless  liquid  of  an  acid  reaction,  and  a  spe- 
cific gravity  of  1005  to  1009.  Chemically  it  contains  per 
thousand  about  973  parts  water  and  27  solids.  Proteid  sub- 
stances compose  some  17  of  the  27  parts  of  solid  matter. 
These  substances  are  mainly  mucin,  pepsin  and  rennin.  The 
most  important  non-nitrogenous  constituent  is  free  hydro- 
chloric acid.  The  others  are  chiefly  the  chlorides  of  sodium, 
potassium,  calcium,  and  ammonium,  and  the  phosphates  of 
iron,  calcium,  and  magnesium,  The  amount  of  gastric  juice 


GASTRIC  GLANDS  91 

secreted  in  twenty-four  hours  is  from  six  to  fourteen  pounds. 
Gastric  juice  will  resist  putrefaction  for  a  long  time,  prob- 
ably on  account  of  the  free  acid.  Its  digestive  properties 
are  due  to  the  proteolytic  enzyme,  pepsin,  the  milk-curdling 
enzyme  rennin,  and  the  free  hydrochloric  acid. 

Hydrochloric  Acid — The  amount  of  free  hydrochloric 
acid  present  in  normal  gastric  juice  is  from  two-tenths  to 
three-tenths  of  one  per  cent.  It  has  been  frequently  claimed 
that  the  acidity  of  this  secretion  is  due  to  lactic  acid,  but 
while  it  cannot  be  denied  that  lactic  acid,  from  the  fermenta- 
tion of  carbohydrates  is,  or  may  be,  normally  in  the  stomach 
during  ingestion,  yet  hydrochloric  acid  is  undoubtedly  the 
free  acid  proper  to  the  gastric  juice.  Digestion,  however, 
will  proceed  under  a  proper  (variable)  degree  of  an  acidity 
from  almost  any  acid. 

Beyond  an  insignificant  effect  in  converting  cane  sugar 
into  dextrose,  its  function  is  a  passive  one,  namely,  that  of 
simply  making  the  secretion  acid,  so  that  pepsin  may  act  upon 
the  proteids. 

Pepsin. — Pepsin  is  a  proteolytic  enzyme,  the  composition 
of  which  has  not  been  determined.  From  the  definition,  it 
converts  proteids  into  peptones.  It  operates  only  in  an  acid 
medium.  Hence  its  action  is  contingent  upon  the  presence 
of  another  constituent  of  the  gastric  juice,  namely,  hydro- 
chloric acid.  Pepsin  is  a  typical  enzyme,  and  reference  to 
the  characteristics  of  those  bodies  will  avoid  repetition  of  its 
properties  here. 

Rennin. — Rennin  has  the  property  of  coagulating  milk. 
It  acts  upon  the  soluble  proteid  of  milk  (casein),  changing  it 
into  an  insoluble  product,  which  is  precipitated.  Acids  also 
will  coagulate  casein.  Milk  when  left  standing  at  ordinary 
temperature  has  lactic  acid  produced  by  the  action  of  bac- 
teria upon  the  lactose  in  it,  and  this  acid  precipitates  the 
curd.  The  acid  of  the  gastric  juice  might  be  sufficient  to 
bring  about  this  result,  but  the  quick  coagulation  of  milk 
when  it  is  introduced  into  the  stomach  is  probably  not  due 


92  THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

to  the  acid,  since  neutral  extracts  of  the  gastric  mucous  mem- 
brane will  themselves  curdle  milk.  After  coagulation  the 
action  of  pepsin  begins  and  the  casein  is  converted  into  pep- 
tones in  the  usual  manner.  The  value  of  the  curdling  pro- 
cess is  not  apparent. 

Action  of  Gastric  Juice  on  Foods.  (A)  On  Proteids. — A 
familiar  test  for  the  proper  performance  of  gastric  digestion 
is  the  observation  of  the  effect  of  the  juice  in  a  given  case 
upon  the  white  of  an  egg  (proteid).  In  normal  gastric 
juice,  or  in  a  properly  prepared  artificial  solution,  the  egg  is 
seen  to  swell  up  and  dissolve.  This  soluble  proteid  is  now 
called  peptone,  and  it  differs  from  the  proteid  of  the  egg  in 
certain  important  respects,  to  be  noted  later.  .But,  although 
peptone  is  the  final  product  of  pepsin-hydrochloric  action, 
there  are  certain  substances  produced  intermediate  between 
the  initial  proteid  and  the  final  peptone,  just  as  in  the  case 
of  the  formation  of  maltose  by  ptyalin.  Some  of  these  sub- 
stances have  been  called  acid-albumin,  parapeptone,  propep- 
tone,  etc.  But  whatever  they  may  be,  the  nomenclature  of 
Kuhne  is  being  largely  followed  at  present.  He  supposes 
that  the  first  product  is  an  acid  albumin  which  he  calls  syn- 
tonin ;  that  syntonin  under  the  influence  of  pepsin  undergoes 
hydrolysis,  taking  up  water  and  splitting  into  primary  pro- 
teoses;  that  each  of  these  primary  proteoses  takes  up  water 
and  splits  into  secondary  proteoses;  that  these  last  undergo 
a  similar  change  with  the  production  of  peptones;  so  that 
the  successive  substances  are  proteid,  syntonin  (acid-albu- 
min), primary  proteoses,  secondary  proteoses,  peptones. 

Peptones  can  be  shown  to  be  different  from  syntonin  and 
the  proteoses  by  chemical  reaction.  The  chief  object  of  pro- 
teolytic  digestion  is  to  get  the  proteids  into  a  diffusible 
condition.  Peptones  differ  from  proteids  in  at  least  three  im- 
portant respects:  (i)  They  can  pass  through  animal  mem- 
branes, that  is,  can  be  absorbed;  (2)  they  are  no  longer  co- 
agulable  by  heat  or  many  acids;  (3)  they  are  capable  of  as- 
similation by  the  cells  after  they  have  been  absorbed. 


GASTRIC  GLANDS  93 

(B)  On  Carbohydrates. — There  is  no  enzyme  furnished 
by  the  stomach  to  affect  any  of  the  carbohydrates.  It  is 
true  that  salivary  digestion  proceeds  in  some  small  degree 
in  the  stomach.  Saliva  is  swallowed  with  the  food,  and  until 
the  reaction  becomes  acid  (which  cannot  be  immediately), 
there  is  no  reason  why  the  conversion  of  starch  into  maltose 
should  not  proceed.  It  is  also  true  that  the  mere  acid  of  the 
gastric  juice  can  slowly  convert  cane  sugar  into  dextrose. 
Simple  acidulated  water  will  do  the  same. 
•  (C)  On  Fats. — Neither  is  there  any  fat-splitting  enzyme 
in  the  gastric  secretion.  So  far  as  any  chemical  change  is 
concerned  the  fats  leave  the  pylorus  in  exactly  the  same  con- 
dition as  they  entered  the  mouth.  Their  physical  condition, 
however,  undergoes  some  change  in  the  stomach.  The  body 
temperature  is  sufficient  to  liquefy  them,  the  vesicles  in 
which  the  droplets  are  contained  are  dissolved,  and  thus  set 
free,  they  become  a  part  of  the  mechanical  mixture,  chyme, 
and  are  made  easier  subjects  of  intestinal  digestion. 

(D)  On  Albuminoids. — The  albuminoids  are  acted  upon 
by  pepsin  and  hydrochloric  acid  in  much  the  same  way  as  are 
the  proteids.  Taking  gelatin  as  a  type,  gelatoses  are  formed 
instead  of  proteoses.  It  is  stated  that  peptic  digestion  does 
not  go  further  than  the  gelatose  stage  with  the  albuminoids, 
conversion  into  peptones  taking  place  under  the  influence  of 
trypsin. 

Resistance  of  Stomach  Wall  to  Digestion. — It  would  be 
interesting  to  know  why  the  stomach  (or  the  intestine)  does 
not  digest  itself.  If  a  portion  of  the  stomach  of  another 
animal  be  placed  in  that  of  a  living  animal,  it  will  be  di- 
gested ;  or  if  the  circulation  be  cut  off  from  a  limited  area  of 
the  stomach,  the  secretion  will  frequently  digest  that  part  of 
the  organ  and  bring  about  a  perforation ;  or  further,  if  any 
living  part  of  an  animal,  as  the  leg  of  a  frog,  be  fastened  in 
the  stomach  of  another  animal,  it  will  likewise  be  digested. 
The  last  instance  would  seem  to  lead  to  the  conclusion  that 
living  matter  can  be  digested,  but  in  reality  it  is  shown  (Ber- 


94  THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

nard)  that  the  tissue  is  first  killed  by  the  acid,  and  that  no 
digestion  takes  place  in  the  alkaline  intestinal  juice.  But 
why  the  stomach  is  not  thus  attacked  when  other  living  tissue 
is  remains  obscure.  The  most  plausible  theory  is  that  the 
gastric  epithelium  is  possessed  of  some  power,  mechanical 
^or  physical,  the  nature  of  which  is  unknown,  inhibiting  the 
action  of  the  gastric  juice,  most  probably  by  preventing  its 
absorption. 

"A  nearer  approach  to  an  explanation  seems  to  have  been 
attained  in  the  discovery  of  an  antipeptic  and  antitryptic  ac- 
tion of  the  stomach  and  intestinal  mucosa.  This  action  is 
probably  due  to  antienzymes  which  are  found  throughout 
the  whole  animal  scale  and  occur  not  only  in  the  intestinal 
tract,  but  also  in  cells  of  other  organs. "  (Tigerstadt.) 

Movements  of  the  Stomach. — Whether  the  exact  details  of 
the  muscular  movements  of  the  stomach  be  known  or  not,  the 
essential  fact  to  be  remembered  is  that  the  organ  is  in  a  more 
or  less  continuous  state  of  muscular  activity  for  several 
hours  after  the  ingestion  of  an  ordinary  meal,  and  that  this 
activity  results  in  the  physical  disintegration  of  most  of  the 
solids  introduced,  in  the  thorough  mixing  of  all  classes  of 
foods  with  each  other  and  with  the  gastric  juice,  and  in  the 
passage  from  time  to  time  of  such  parts  as  have  been  re- 
duced to  a  pultaceous  condition  through  the  pylorus  into  the 
duodenum,  until  finally  the  stomach  is  empty. 

In  considering  the  mechanism  of  these  movements  a  di- 
vision of  the  organ  into  two  segments,  fundic  and  pyloric,  by 
the  sphincter  antri  pylorici  is  to  be  kept  in  mind.  When  food 
has  entered  the  stomach  the  peristaltic  wave  of  contraction 
begins  at  the  splenic  end  and  passes  toward  the  right.  This 
contraction  is  comparatively  weak,  is  mainly  evident  along 
the  greater  curvature,  and  increases  in  strength  as  it  passes 
toward  the  pylorus.  Its  wave-like  character  is  due  to  the 
contraction  and  subsequent  relaxation  of  successive  bands  of 
circular  and  oblique  fibers.  Regurgitation  of  food  is  pre- 
vented by  a  rhythmical  contraction  of  the  lower  end  of  the 


GASTRIC  GLANDS  95 

esophagus,  and  the  effect  of  this  muscular  wave  (peristalsis) 
in  the  fundus  is  to  force  the  food  toward  the  pylorus.  But 
when  the  right  end  is  reached,  the  rather  firm  contraction  of 
the  sphincter  antri  pylorici  prevents  the  entrance  into  the  an- 
trum  of  all  except  the  liquid  or  semi-liquid  parts.  The  food, 
thus  denied  admission  to  the  antrum,  takes  a  course  along  the 
lesser  curvature  to  the  splenic  end,  then  back  along  the 
greater  curvature,  and  such  parts  of  it  as  have,  during  this 
revolution,  been  sufficiently  dissolved  pass  into  the  antrum. 
These  revolutions  continue  until  the  fundus  has  been  emp- 
tied. 

It  is  not  to  be  supposed  that  food  has  been  accumulating 
meantime  in  the  antrum.  Indeed,  it  is  certain  that  muscular 
contractions  are  here  much  more  active  than  in  the  fundus, 
where  the  movements  are  slow  and  of  a  rather  compressing 
nature.  It  is  thought  that  very  soon  after  the  entrance  of 
food  from  the  fundus  the  entire  muscular  wall  of  the  antrum 
undergoes  very  strong  contraction  of  a  peristaltic  nature,  and 
the  pultaceous  parts  of  its  contents  are  sent  with  some  force 
into  the  duodenum.  Those  which  are  not  sufficiently  dis- 
solved to  pass  the  pyloric  sphincter  are  said  to  excite  an  anti- 
peristaltic  movement,  whereby  they  are  thrown  back  into  the 
fundus  for  further  digestion — the  sphincter  antri  pylorici 
having  now  relaxed.  However,  substances  which  the  gastric 
juice  and  contractions  cannot  dissolve  will  finally  pass  the 
pylorus,  but  they  are  probably  delayed  for  a  considerable 
time. 

This  succession  of  movements  is  continued  with  a  rapidity 
and  regularity  varying  with  the  condition  of  the  organ  and 
the  nature  of  its  contents.  They  last  until  the  organ  is  emp- 
tied in  part  by  the  absorption  of  its  contents,  but  mainly  by 
their  passage  into  the  small  intestine.  Each  circuit  in  the 
fundus  probably  occupies  about  three  minutes,  and  gastric 
digestion  as  a  whole  lasts  usually  from  two  to  five  hours. 
The  contraction  and  relaxation  of  plain  muscle  is  much 
slower  than  that  of  striped. 


96          THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

It  is  the  fundus,  and  not  the  pylorus,  which  serves  as  a 
reservoir  and  in  which  the  greater  part  of  gastric  digestion 
occurs.  The  precise  condition  of  the  pyloric  sphincter  dur- 
ing gastric  digestion  is  unknown.  It  may  have  simply  an  ex- 
alted degree  of  tonicity  which  does  not  completely  close  the 
opening  and  which  can  be  overcome  by  pressure,  or  it  may  be 
tightly  contracted  and  require  a  distinct  nervous  dispensa- 
tion to  effect  its  relaxation  for  the  passage  of  fluids  as  well 
as  solids.  It  would  seem  that  the  length  of  time  for  which 
food  is  detained  in  the  stomach  depends  more  upon  its  phy- 
sical condition  than  upon  its  chemical — that  is,  that  upon 
any  stage  of  digestion  which  it  may  have  reached ;  for  it  can 
be  shown  that  fluids  pass  very  quickly  into  the  intestine. 

The  secretory  occurrences  during  these  movements  are  of 
the  greatest  importance  (see  pp.  86-88). 

Nerve  Supply. — The  stomach  is  supplied  with  pneumo- 
gastric  and  sympathetic  fibers.  The  latter  can  be  traced 
through  the  solar  plexus,  splanchnics  and  cervical  ganglia  to 
the  spinal  cord.  They  exert  an  inhibitory  effect  on  the  mus- 
cular tissues;  their  stimulation  causes  relaxation.  The 
vagus  fibers  are  motor ;  their  stimulation  causes  contraction. 
But  these  nerves  serve  only  to  regulate  the  muscular  move- 
ments. It  is  the  stimulus  of  food  in  the  stomach  which  ex- 
cites gastric  peristalsis.  It  is  not  stopped  by  section  of  the 
nerves,  though  it  may  be  interfered  with.  This  stimulation 
is  exerted  either  directly  upon  the  nerve  fibers  or  upon  the 
ganglia  of  the  stomach  wall. 

The  conditions  influencing  gastric  digestion  operate  mainly 
through  changes  in  the  quality  and  quantity  of  gastric  juice. 

Digestion  and  Absorption  in  the  Intestines. 
The  Small  Intestine. 

Anatomy. — The  small  intestine  extends  from  the  pylorus 
to  the  caput  coli,  and  is  about  twenty  feet  in  length.  It  lies 
in  numerous  coils  which  are  held  loosely  in  place  by  a  fold  of 


DIGESTION  AND  ABSORPTION   IN  THE  INTESTINES  97 

peritoneum  running  from  one  side  of  the  great  abdominal 
vessels,  enveloping  the  gut,  and  returning  to  the  parietal  wall 
on  the  opposite  side  of  the  vessels.  The  fold  thus  attaching 
the  intestine  to  the  abdominal  wall  is  the  mesentery.  The 
distance  along  the  mesentery  from  this  parietal  region  to  the 
gut  is  three  or  four  inches,  except  at  the  beginning  and  end  of 
the  small  intestine,  where  it  is  shorter,  to  bind  the  tube  more 
firmly  in  place.  The  upper  eight  or  ten  inches  of  the  small 
gut  is  called  the  duodenum,  the  next  eight  feet  the  jejunum, 
and  the  remainder  the  ileum.  No  anatomical  peculiarity  sep- 
arates these  parts.  The  average  diameter  is  about  one  and  a 
quarter  inches. 

Histology. — The  wall  of  the  intestine  is  in  four  layers, 
serous,  muscular,  submucous  and  mucous.  The  serous  layer 
consists  of  the  enveloping  fold  of  peritoneum  and  needs  no 
description,  except  that,  like  serous  membranes  elsewhere,  it 
furnishes  a  lubricating  secretion  to  provide  for  the  easy  glid- 
ing of  the  intestines  over  each  other  and  over  the  other  vis- 
cera. The  muscular  coat  has  its  muscular  fibers  disposed  in 
two  layers,  an  external  longitudinal  and  an  internal  circular. 
The  latter  is  the  stronger.  Between  the  two  muscular  layers 
is  the  nervous  plexus  of  Auerbach.  Between  the  circular 
layer  and  the  mucous  coat  is  the  submucous  layer  which  con- 
tains the  nerve  plexus  of  Meissner.  These  communicate 
with  others  by  fibers  of  extension.  The  mucous  coat  pre- 
sents several  points  deserving  mention.  These  are  (i)  val- 
vulse  conniventes;  (2)  villi ;  (3)  secreting  glands,  (a)  of 
Brunner  and  (b)  of  Lieberkuhn;  (4)  solitary  and  agminate 
glands. 

i .  The  valvulae  conniventes  are  simply  tpansverse  folds  or 
tucks  of  the  entire  mucous  membrane,  each  of  which  extends 
from  one-third  to  one-half  around  the  circumference  of  the 
tube  and  projects  by  its  middle  portion  sometimes  to  the  cen- 
ter of  the  lumen.  The  small  folds,  800  to  1,000  in  number, 
extend  from  about  the  middle  of  the  duodenum  to  the  begin- 
ning of  the  last  third  of  the  ileum  and  greatly  increase  the 


98          THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

length  of  the  mucous  membrane  over  that  of  the  gut  proper. 
They  are  not  effaced  by  the  passage  of  food  or  by  other  cir- 
cumstances, for  the  two  surfaces  of  the  fold  which  are  in  ap- 
position are  bound  together  by  loose  connective  tissue.  The 
fold  as  a  whole,  however,  is  freely  movable  upward  or  down- 
ward in  the  intestine  and  has  no  tendency  to  obstruct  the 
canal.  The  only  function  of  the  valvulae  conniventes  is  to 
furnish  a  greater  secreting  surface  and,  by  somewhat  re- 


FIG.  39. — Diagram  of  a  longitudinal  section  of  the  wall  of  the  small 

intestine. 

a,  villi;  b,  Lieberkuhn's  glands;  c,  tunica  muscularis  mucosae,  below  which 
lies  Meissner's  nerve  plexus;  d,  connective  tissue  in  which  many  blood  and 
lymph  vessels  lie;  e,  circular  muscle  fibers  cut  across  with  Auerbach's  nerve 
plexus,  below  it;  f,  longitudinal  muscle  fibers;  g,  serous  coat.  (Yeo.) 

tarding  the  passage  of  the  alimentary  mass,  to  subject  it  for 
a  longer  time  to  the  digestive  fluids. 

2.  The  villi  are  especially  important  in  connection  with 
absorption,  and  their  description  properly  belongs  under  that 
head.  They  are  conical  elevations  responsible  for  the  velvety 
character  of  the  'mucous  membrane.  They  exist  in  great 
numbers  from  the  pylorus  to  the  ileo-cecal  valve,  covering 
the  valvulse  conniventes  as  well  as  the  general  surface  of  the 
mucous  membrane.  The  largest  are  about  ^o  in.  long  and  ^o 
in.  in  diameter  at  their  base.  They  are  only  elevations  of  the 
mucous  membrane  containing  a  central  tube,  the  lacteal, 
which  is  nothing  but  an  intestinal  lymphatic.  The  structure 


DIGESTION  AND  ABSORPTION  IN  THE  INTESTINES 


99 


from  without  inward — that  is,  from  the  surface  of  the  villus 
inward  to  its  center — is  (i)  a  layer  of  columnar  epithelium 
resting  upon  a  delicate  basement  membrane;  (2)  lymphoid 


FIG.  40. — Portion  of  the  wall  of  the  small  intestine  laid  open  to 
show  the  valvulae  conniventes.      (From   Yeo  after  Brinton.) 

tissue  containing  abundant  capillaries  and  connective  tissue 
cells;  (3)  a  thin  layer  of  plain  muscle  fibers  continuous  from 


FIG.  41.— Vertical  section  of  a  villus  of  the  small  intestines  of  a  cat. 

a,  striated  border  of  the  epithelium;  b,  columnar  epithelium;  c,  goblet  cells; 
d,  central  lymph-vessel;  e,  smooth  muscular  fibers;  f,  adenoid  stroma  of  the 
villus  in  which  lymph  corpuscles  lie.  (Kirkes  after  Klein.) 

the  intestinal  wall;  (4)  the  lacteal,  whose  endothelial  wall 
contains  many  stomata. 

3.  The  glands  of  Brunner  and  the  crypts  of  Lieberkuhn, 
or  intestinal  tubules,  are  supposed  to  produce  the  succus  en- 


IOO        THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

tericus.  The  former  are  chiefly  limited  to  the  upper  half  of 
the  duodenum.  The  latter  exist  throughout  the  small  and 
large  intestine. 

4.  The  solitary  and  agminate  glands  are  not  supposed  to 
contribute  to  the  production  of  the  intestinal  juice.  They  are 
alike  in  structure,  the  agminate  glands  being  only  a  collection 
of  solitary  glands.  The  former  are  the  Peyer's  patches,  so 
important  in  the  pathology  of  typhoid  fever.  These  patches 
are  usually  about  twenty  in  number  and  confined  to  the  lower 
two-thirds  of  the  ileum,  where  they  occupy  that  portion  of 
the  circumference  of  the  tube  opposite  the  attachment  of  the 
mesentery.  Their  average  dimensions  are  I  X  il/2  in.  They 
consist  essentially  of  lymphoid  tissue,  the  separate  follicles  of 
which  are  surrounded  by  lymphatics  and  penetrated  by 
blood-vessels.  They  are  covered  by  villi,  but  the  valvulae 
conniventes  cease  at  their  edges.  The  solitary  glands  are 
more  widely  distributed  than  the  agminate. 

The  chyme,  having  passed  from  the  stomach  to  the  small 
intestine,  encounters  three  digestive  fluids,  pancreatic  juice, 
bile  and  intestinal  juice.  These  are,  of  course,  mixed  to- 
gether, but  none  interferes  with  the  action  of  the  other. 

The  Pancreas. — The  pancreas  is  a  large  gland  lying  in  the 
upper  part  of  the  abdominal  cavity  behind  the  stomach.  It 
has  the  general  shape  of  a  hammer,  its  head  being  embraced 
by  the  bend  of  the  duodenum  and  its  opposite  extremity 
reaching  to  the  spleen.  It  weighs  some  four  or  five  ounces, 
and  is  about  seven  inches  long.  Its  duct,  the  duct  of  Wir- 
sung,  usually  joins  the  common  bile  duct  just  where  this  lat- 
ter penetrates  the  wall  of  the  duodenum,  so  that  the  bile  and 
pancreatic  juice  enter  the  small  intestine  together.  Some- 
times the  two  ducts  do  not  join,  and  sometimes  a  second 
smaller  duct  from  the  pancreas  penetrates  the  duodenum  a 
little  below  the  larger  one.  The  duct  of  Wirsung  traced 
backward  divides  and  subdivides  until  its  final  ramifications 
end  in  the  alveoli,  or  secreting  portions. 

Histology. — This  is  a  compound  tubular  gland.    The  cells 


THE  PANCREAS  IOI 

in  the  alveoli  are  of  the  serous  type  aiid-'sxe  gr&nular  V>\v;ird 
the  central  lumen.  During  activity  they  undergo-  .changes 
very  similar  to  the  salivary  cells;  Ihe'il6il->&#nuter,'£&ite  to- 
ward the  basement  membrane  increasing  and  extending  and 
the  granular  zone  becoming  correspondingly  smaller.  Here, 
as  in  the  salivary  glands,  it  is  believed  that  the  granules  are 
made  from  the  clear  protoplasm,  and  contain  the  enzymes  or 


a 
A 

FIG.  42. — One  sacctile  of  the  pancreas  of  the  rabbit  in  different  states 
of  activity.     (From  Brubaker  after  Yeo.) 

A,  after  a  period  of  rest,  in  which  case  the  outlines  of  the  cells  are  indistinct 
and  the  inner  zone — i.  e.,  the  part  of  the  cells  (a)  sext  the  lumen  (c) — is  broad 
and  filled  with  fine  granules.  B,  after  the  gland  has  poured  out  its  secretion, 
when  the  cell  outlines  (d)  are  clearer,  the  granular  zone  (a)  is  smaller,  and  the 
clear  outer  zone  is  wider. 

their  formative  materials.  The  formative  material  in  all 
these  glands  is  given  the  name  of  zymogen,  although  the  zy- 
mogen  in  a  particular  gland  may  have  a  particular  name,  as 
pepsinogen,  the  forerunner  of  pepsin,  or  trypsinogen,  the 
forerunner  of  trypsin. 

Properties  and  Composition  of  Pancreatic  Juice. — The 
pancreatic  juice  is  a  colorless  liquid,  alkaline  in  reaction,  and 
has -a  specific  gravity  of  about  1040  if  taken  from  a  recent 
fistula.  It  coagulates  when  heated  and  is  prone  to  putrefac- 
tion on  exposure.  With  a  specific  gravity  of  about  1040,  it 
contains  per  thousand  about  900  parts  of  water,  the  remain- 
der being  different  solid  food  materials  in  solution.  These 
constituents  are  a  proteid  and  three  very  important  digestive 


IO2        THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

f  erments,  tryp&ift,  steapsin  and  amylopsin.  The  phosphates 
and  carbonates  are  plentiful  and  give  the  fluid  its  alkaline 
reaction. 

Trypsin. — Trypsin,  like  pepsin,  converts  proteids  into  pep- 
tones. Nothing  positive  is  known  of  its  composition,  but  it 
is  possessed  of  the  usual  characteristics  of  enzymes  regard- 
ing temperature,  etc.  It  differs  from  pepsin  in  that  its  pro- 
teolytic  action  is  more  powerful  and  can  take  place  in  alka- 
line media.  It  will  also  act  in  neutral  or  weakly  acid  media. 
The  opinion  is  advanced  that  while  the  gastric  juice  is  capa- 
ble of  converting  proteids  into  peptones,  as  a  matter  of  fact 
it  does  not  usually  carry  the  process  further  than  the  pro- 
teose  stage,  and  thus  prepares  the  proteoses  for  tryptic  di- 
gestion. 

It  was  seen  that  the  successive  products  of  pepsin-hydro- 
chloric digestion  are  syntonin,  primary  proteoses,  secondary 
proteoses  and  peptones.  In  tryptic  digestion  it  seems  that,  in 
the  splitting  process,  the  syntonin  (here  alkali-albumin)  and 
primary  proteose  stages  are  omitted,  and  the  first  product  is 
secondary  proteoses,  which  are  split  into  peptones.  Further- 
more trypsin  goes  a  step  beyond  with  some  of  the  peptones 
and  converts  them  into  simpler  compounds,  the  best  known 
of  which  are  leucin  and  tyrosin.  These  are  found  normally 
in  the  intestinal  canal,  but  the  physiological  importance  of 
this  conversion  is  not  apparent.  The  opinion  that  it  is  a 
useless  sacrifice  of  useful  peptones  does  not  seem  warranted. 

Amylopsin. — The  amylolytic  enzyme,  amylopsin,  is  iden- 
tical in  its  action  with  ptyalin.  This  enzyme  is  very  impor- 
tant, for  it  has  been  remarked  that  the  action  of  ptyalin  is 
probably  rather  inconsequential,  and  by  far  the  greater  por- 
tion of  the  starch,  which  constitutes  a  large  part  of  our  ordi- 
nary food,  must  be  digested  in  the  small  intestine — and  al- 
most entirely  by  amylopsin. 

Steapsin. — Under  the  influence  of  steapsin  neutral  fats 
take  up  water  and  undergo  hydrolysis,  with  the  production 
of  glycerine  and  the  fatty  acid  corresponding  to  the  kind 


INTERNAL  PANCREATIC  SECRETION  IO3 

of  fat  which  is  split  up.  In  the  intestine  it  is  probable  that 
only  a  part  of  the  neutral  fats  are  thus  split  in  glycerine  and 
fatty  acids.  The  fatty  acids  thus  formed  unite  with  the  alka- 
line salts  to  form  soaps,  and  these  soaps,  aided  by  intestinal 
peristalsis,  convert  the  remaining  fats  into  an  emulsion.  The 
products  of  fat  digestion  are  therefore  glycerine,  soaps,  and 
emulsions,  all  of  which  can  be  absorbed  in  a  way  to  be  noted 
later.  While  the  emulsification  of  fats  under  the  influence  of 
soaps  (fatty  acids  and  alkaline  salts)  is  an  undoubted  effect, 
the  method  of  procedure  is  unknown.  It  is  certain  that  the 
emulsification  is  aided  by  the  presence  of  bile,  although  this 
fluid  possesses  no  fat-splitting  enzyme. 

Method  of  Secretion. — It  can  be  shown  that  the  secretion 
begins  to  be  discharged  into  the  duodenum  very  soon  after 
the  entrance  of  food  into  the  stomach,  and  continues  as  long 
as  intestinal  digestion  is  in  progress.  Consequently  the  flow 
will  be  intermittent  if  the  meals  are  far  enough  apart.  It  is 
almost  certain  that  the  secretion  is  a  reflex  act  as  a  result  of 
impressions  upon  the  mucous  membrane  of  either  the  stom- 
ach or  duodenum.  The  acidity  of  the  gastric  juice  seems  to 
be  the  natural  stimulus  and  to  exert  its  influence  upon  the 
duodenal  mucous  membrane.  This  is  not  incompatible  with 
the  early  flow  after  the  ingestion  of  food,  for  it  will  be  seen 
later  that  at  least  a  small  quantity  of  that  food  passes  quickly 
to  the  duodenum  and  carries  gastric  juice  with  it.  The  com- 
position of  the  secretion  seems  to  be  influenced  in  some  de- 
gree by  the  character  of  the  food.  It  is  interesting  that  oils 
increase  the  pancreatic  flow. 

Nerve  Supply. — The  pancreas  has,  besides  vaso-motor 
fibers  to  its  vessels,  distinct  secretory  fibers,  like  those  of  the 
salivary  glands.  These  fibers  probably  run  in  both  the  sym- 
pathetic and  the  vagus. 

Internal  Pancreatic  Secretion. — Circumstantial  evidence 
leaves  scarcely  any  doubt  that  the  pancreas  produces  some 
substance  which  is  discharged  into  the  blood  and  markedly 
influencees  nutrition.  Removal  of  the  gland  is  followed  by 


IO4        THE-  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

death  from  inanition  in  two  or  three  weeks ;  and  previous  to 
that  sequel  the  most  striking  phenomenon  is  marked  glyco- 
suria,  with  the  ordinary  symptoms  of  diabetes  mellitus.  Re- 
tention of  a  comparatively  small  portion  of  the  gland  obviates 
this  condition.  Sugar  does  not  exist  normally  in  the  blood, 
and  this  internal  secretion  may  contain  some  ferment  which 
effects  its  consumption. 

The  Liver. 

The  liver  is  the  largest  gland  in  the  body.    Its  function  is 
to  produce  bile,  glycogen  and  urea. 


FIG.  43. — The  under  surface  of  the  liver. 

i,  lobus  hepatis  sinister;  2,  lobus  henatis  dexter;  3,  quadrate  lobe;  4,  caudate 
lobe;  5,  lobus  caudatus;  6,  hepatic  artery;  7,  portal  vein;  8,  fossa  ductus  venosi; 
9,  fossa  vesicae  fellae;  10,  cystic  duct;  n,  hepatic  duct;  12,  fossa  venae  cavae; 
13,  vena  cava. 

Anatomy. — The  liver  is  situated  in  the  upper  part  of  the 
abdominal  cavity,  chiefly  in  the  right  hypochondrium.  Its 
weight  in  the  average  adult  is  about  four  and  a  half  pounds. 
It  is  covered,  except  for  a  small  area  behind,  by  peritoneum, 
processes  of  which  run  from  it  at  several  points  and  consti- 
tute its  supporting  ligaments.  The  proper  coat  of  the  liver 
lies  underneath  the  peritoneum,  and  at  the  transverse  fissure 


THE  LIVER  IO5 

is  continued  into  the  gland  as  a  sheath,  embracing  the  struc- 
tures entering  there  and  ramifying  with  them  in  their  distri- 
bution. This  is  the  capsule  of  Glisson.  It  is  fibrous  in  struc- 
ture, is  closely  attached  to  the  liver  substance,  and  rather 
loosely  adherent  to  the  structures  which  it  envelops.  The 
walls  of  the  portal  vein  are  seen  collapsed  on  section,  while 
those  of  the  hepatic  veins,  which  are  not  surrounded  by  Glis- 
son's  capsule,  and  which  are  closely  adherent  to  the  gland 
substance,  stand  well  open. 

A  general  idea  of  the  liver's  anatomy  is  obtained  by  noting 
that  it  has  five  lobes,  five  fissures,  five  ligaments  and  five 
structures  passing  through  the  transverse  fissure.  The  lobes 
are  right,  left,  caudate,  quadrate  and  Spigelian.  The  fissures 
are  transverse,  umbilical,  that  for  the  ductus  venosus,  the 
fossa  for  the  vena  cava  and  the  fossa  vesicalis.  The  liga- 
ments are  coronary,  right  lateral,  left  lateral,  round  and  sus- 
pensory or  longitudinal.  The  structures  passing  through  the 
transverse  fissure  are  the  portal  vein,  the  hepatic  artery,  the 
hepatic  duct,  the  nerves  and  the  lymphatics. 

Blood-vessels. — Of  the  two  blood-vessels  entering  the  fis- 
sure the  portal  vein  is  decidedly  the  larger.  It  has  collected 
the  blood  from  the  abdominal  organs  by  the  radicles  of  its 
tributaries,  the  gastric,  splenic,  superior  and  inferior  mesen- 
teric  veins,  while  the  hepatic  artery  is  a  branch  of  the  celiac 
axis.  These,  having  been  distributed  in  a  manner  to  be  noted 
presently,  discharge  their  blood  into  the  radicles  of  the 
hepatic  veins,  which,  usually  three  in  number,  enter  the  as- 
cending vena  cava,  where  that  vessel  passes  through  the  liver 
behind.  Again,  it  is  to  be  remembered  that  these  two  vessels, 
as  well  as  the  nerves  and  lymphatics,  are  enveloped  in  the 
vagina,  or  capsule  of  Glisson. 

The  portal  vein  and  the  hepatic  artery  give  off  branches  to 
the  capsule  of  Glisson,  constituting  the  vaginal  plexus.  The 
portal  vein,  still  ensheathed,  then  divides  and  subdivides  until 
its  branches  run  directly  between  the  lobules,  and  are  called 
interlobular  veins.  These  direct  subdivisions  of  the  portal 


106        THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

vein  are  not  the  only  interlobular  veins,  however.  Those 
branches  of  this  vein  which  were  given  off  to  the  capsule  of 
Glisson,  having  received  the  corresponding  branches  from 
the  hepatic  artery,  also  here  run  between  the  lobules  and 
make  part  of  the  interlobular  plexus.  The  interlobular  veins, 
thus  surrounding  the  lobules  and  having  lobules  on  either 


FIG.  44. — Diagram  of  the  portal  vein. 

(pv)   arising  in  the  alimentary   tract   and   spleen    (s)    and   carrying  the  blood 
from  these  organs  to  the  liver.     (From  Brubaker  after  Yeo.) 

side  of  them,  giving  off  in  both  directions  branches  (lobular 
branches)  which  penetrate  the  lobules,  to  break  up  into  ca- 
pillaries. The  capillaries  finally  converge  to  three  or  four 
small  radicles,  which  in  turn  unite  to  form  a  small  vein  in  the 
center  of  the  lobule.  This  is  the  intralobular  vein,  which  at 
the  base  of  the  lobule  joins  the  sublobular  vein.  These  sub- 
lobular  veins  join  each  other  to  form  hepatic  veins,  which 


THE  LIVER 


107 


become  larger  and  larger  until  they  have  collected  all  the 
blood  which  has  entered  the  liver.  They  finally  enter  the 
ascending  vena  cava. 


FIG.  45. — Section  of  lobule  of  liver  of  rabbit  in  which  the  blood 
capillaries  and  bile  canaliculi  have  been  injected.  (From  Yeo  after 
Cadiat.} 

a,  intralobular  vein;  b,  interlobular  veins;  c,  biliary  canals  beginning  in  fine 
capillaries. 

But  what  has  become  of  the  hepatic  artery?  As  soon  as  it 
has  entered  the  sheath,  it  gives  off  branches  to  the  capsule 


IO8        THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

forming  part  of  the  vaginal  plexus  and  entering  into  the  vag- 
inal branches  of  the  portal  vein  just  before  these  run  be- 
tween the  lobules.  It  also  furnishes  branches  to  the  wall  of 
the  portal  vein,  to  the  wall  of  the  larger  divisions  of  the  ar- 
tery itself,  and  to  the  hepatic  duct. 

Histology  of  a  Lobule. — The  liver  is  made  up  of  a  large 
number  of  lobules  about  one-t wenty-.fi fth  of  an  inch  in  di- 
ameter, separated  by  vessels,  nerves  and  radicles  of  the  he- 
patic duct.  Such  a  lobule  in  certain  of  the  lower  animals  has 
a  distinct  polygonal  shape,  but  in  man  the  outlines  are  not 
clear.  In  the  lobule  are  the  hepatic  cells,  ovoid  in  shape, 
possessed  of  small  granules  and  one  or  two  nuclei.  They 
are  disposed  in  columns  radiating  from  the  central  intralob- 
ular  vein.  These  cells  belong  to  the  epithelial  type,  and  the 
liver  is  not  essentially  different  from  other  glands,  such  as 
the  salivary,  except  in  the  complexity  of  its  arrangement. 
The  analogy  is  established  by  the  origin  of  the  bile  ducts  in 
the  lobules  between  the  cells. 

Bile  Ducts. — It  is  not  difficcult  to  demonstrate  the  inter- 
lobular  ducts,  but  to  follow  them  as  such  into  the  lobule  is 
less  easy.  However,  there  is  no  doubt  at  all  that  they  do 
originate  between  the  hepatic  cells.  It  is  probable  that  here 
they  have  no  distinct  lining  membrane,  but  are  mere  tubular 
intercellular  spaces,  into  which  the  bile  is  poured  and  car- 
ried into  the  interlobular  duct.  Typically  a  liver  cell  has 
one  of  these  bile  capillaries  on  one  side  and  a  blood  capillary 
on  the  other,  and  while  this  relation  does  not  always  hold 
good,  every  cell  does  communicate  with  both  kinds  of  capil- 
laries. The  interlobular  bile  ducts  consist  of  epithelium  rest- 
ing upon  a  very  thin  basement  membrane.  As  they  increase 
in  size  they  gain  fibrous  inelastic  and  elastic  tissue,  and  the 
largest,  some  non-striated  muscular  elements.  Gradually  as 
the  ducts  become  larger  the  lining  epithelium  changes  from 
the  columnar  to  the  pavement  form.  Mucous  glands  exist  in 
the  largest  ducts.  The  interlobular  ducts  join  each  other  and 
gradually  increase  in  size  as  they  merge  from  all  parts  of  the 


THE  LIVER 


109 


liver,  to  leave  its  substance  in  two  divisions — one  from  the 
right  and  one  from  the  left  lobe.  These  two  unite  to  form 
the  hepatic  duct  which,  running  a  course  of  about  one  and  a 
half  inches,  is  joined  at  an  acute  angle  by  the  cystic  duct 
to  form  the  common  bile  duct,  or  the  ductus  communis 
choledochus.  The  last  penetrates  obliquely  the  duodenal 

Branch  of  portal  vein. 

Large  interlobular  bile  duct. 

Interlobular  connective 


Central  veins 


Central  vein. 


FIG.  46. — From  a  horizontal  section  of  human  liver.     X4°. 

Three  central  veins,  cut  transversely, .  represent  each  a  center  of  as  many 
hepatic  lobules,  that  at  the  periphery  are  but  slightly  denned  from  their  neigh- 
bors. Below  and  to  the  right  of  the  section  the  lobules  are  cut  obliquely  and 
their  boundaries  cannot  be  distinguished.  (From  Stohr.) 

wall  and  discharges  the  bile  into  the  intestine.  The  cystic 
duct  has  its  origin  at  the  apex  of  the  gall  bladder,  and  is 
about  one  inch  long.  The  common  bile  duct  has  an  average 
length  of  three  inches.  (See  Fig.  43.) 

Gall  Bladder. — The  gall  bladder  has  an  oval  shape  with  its 
large  end  forward.  It  is  on  the  under  surface  of  the  liver, 
the  peritoneum  running  over  (or  rather  under)  it.  It  has  a 
mucous  lining  and  the  remainder  of  its  structure  is  chiefly 


I IO        THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

fibrous.  A  little  plain  muscular  tissue  may  exist.  Its  capac- 
ity is  about  one  and  a  half  ounces.  Mucous  glands  are  found 
in  its  lining,  as  in  that  of  the  large  ducts,  and  these  are  re- 
sponsible for  the  mucin  of  the  bile. 

Hepatic  Nerves. — With  regard  to  the  exact  destination  of 
the  nerves  entering  the  liver,  little  is  known.  Evidence  going 
to  establish  the  termination  of  fibers  in  the  cells,  that  is,  the 
existence  of  distinct  secretory  fibers  is  meager.  There  is 
little  doubt  that  secretory  fibers  for  the  glycogenic  func- 
tion of  the  liver  do  exist.  It  is  known  that  fibers  from  the 
vagus,  phrenic  and  solar  plexus  enter  the  fissure,  but  they 
cannot  be  followed  farther  than  the  ramifications  of  Glisson's 
capsule  between  the  lobules.  Of  course,  vaso-motor  fibers 
go  to  the  vessels,  as  elsewhere.  Fibers  acting  similarly  go 
also  to  the  muscular  tissue  of  the  large  ducts  and  of  the  gall 
bladder.  The  contraction  of  the  gall  bladder  is  thought  to  be 
reflex,  afferent  impressions  being  conveyed  by  the  vagus 
from  the  mucous  membrane  of  the  duodenum. 

Hepatic  Lymphatics. — The  lymphatics  are  abundant,  and 
those  not  originating  in  the  connective  tissue  are  thought  to 
originate  by  perivascular  canals  surrounding  the  blood-ves- 
sels of  the  lobules.  The  fact  that  when  the  outflow  of  bile  is 
occluded  it  passes,  not  into  the  vascular,  but  into  the  lym- 
phatic circulation  is  a  curious  circumstance.  It  may  be  due 
to  the  absence  of  a  definite  wall  for  the  intralobular  ducts 
and  their  comparatively  free  communication  with  the  lym- 
phatics in  those  localities. 

Properties  and  Composition  of  Bile. — Human  bile  is  of  a 
dark  greenish-red  color,  has  a  bitter  taste  and  is  practically 
odorless  when  fresh.  It  undergoes  putrefaction  easily,  but 
is  not  coagulable  by  heat.  It  is  viscid,  chiefly  on  account  of 
the  mucin  it  contains.  It  has  an  alkaline  reaction,  and  a  spe- 
cific gravity  of  about  1030.  Besides  water,  which  consti- 
tutes more  than  ninety  per  cent,  of  its  bulk,  it  contains  the 
sodium  salts  of  taurocholic  acid  and  glycocholic  acid  (the 
biliary  salts),  cholesterin,  bilirubin,  lecithin,  fats,  soaps,  mu- 


THE  LIVER  III 

cin  and  various  inorganic  salts,  such  as  sulphates,  carbonates, 
phosphates,  etc.,  and  a  quantity  of  carbon  dioxide.  The 
quantity  of  bile  secreted  in  twenty-four  hours  is  about  two 
and  a  half  pounds. 

In  human  bile  sodium  taurocholate  largely  predominates 
over  glycocholate.  These  are  formed  as  acids  by  the  liver 
cells,  are  absorbed  in  their  passage  down  the  intestine,  and 
are  presumably  those  parts  of  the  bile  which  are  concerned 
in  its  digestive  action,  particularly  in  the  absorption  of  fats. 
So  far  as  these  constituents  are  concerned,  the  bile  is  a  typi- 
cal secretion. 

Cholesterin,  on  the  other  hand,  seems  to  be  simply  re- 
moved from  the  blood  by  the  liver  cells,  and  is  discharged  in 
the  feces,  where,  however,  it  exists  in  a  slightly  changed 
form,  stercorin.  It  is  thought  to  be  held  in  solution  by  the 
bile  acids,  glycocholic  and  taurocholic.  So  far  as  this  con- 
stituent is  concerned,  therefore,  the  bile  is  a  typical  excretion. 
It  is  produced  in  many  of  the  body  tissues,  and  no  function 
has  been  discovered  for  it. 

Bilirubin  is  the  characteristic  coloring  matter  of  the  hu- 
man bile;  that  of  herbivorous  animals  is  biliverdin,  and  a 
little  of  this  latter  is  also  present  in  human  bile.  These  pig- 
ments originate  from  hemoglobin.  It  is  supposed  that  when 
the  red  corpuscles  break  down,  "the  hemoglobin  is  brought 
to  the  liver,  and  then  under  the  influence  of  liver  cells  is  con- 
verted into  an  iron-free  compound,  bilirubin,  or  biliverdin." 
(Howell.) 

The  lecithin  is  probably  an  end  product  of  physiological 
activity  in  the  tissues,  and  is  apparently  an  excretion. 

The  mucin  gives  the  fluid  its  viscid  character. 

The  production  of  bile  is  continuous,  but  this  does  not 
mean  that  its  discharge  into  the  duodenum  is  continuous,  for 
in  the  intervals  of  digestion  it  is  not  admitted  (freely  at 
least)  into  the  intestine,  but  regurgitates  from  the  ductus 
communis  choledochus  through  the  cystic  duct  into  the  gall 
bladder,  which  acts  as  a  reservoir  until  its  contents  are 


112        THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

needed.  The  secretion  is  more  active,  however,  during  in- 
testinal digestion  than  at  other  times.  This  appears  to  be 
reflex,  but  may  be  simply  a  result  of  the  increased  amount  of 
blood  passing  through  the  portal  vein  to  the  liver  during  that 
period,  for  the  whole  alimentary  canal  is  congested  while  di- 
gestive activity  is  in  progress.  Again,  it  is  known  that  the 
best  cholagogue  is  bile  itself,  and  some  of  the  bile  is  ab- 
sorbed in  its  passage  down  the  intestine.  Its  presence  in  the 
blood  may  account  for  the  accelerated  flow. 

Method  of  Secretion  and  Discharge. — The  bile  is  a  pro- 
duct of  the  liver  cells.  How  they  receive  their  normal  stimu- 
lus is  obscure.  But  it  is  reasonable  to  suppose  that  a  larger 
supply  of  blood  means  a  more  abundant  secretion.  Such  an 
increase  of  blood  supply  occurs  during  digestion. 

The  cells  discharge  the  bile  into  the  bile  capillaries,  which 
pass  it  onward  either  to  the  intestine  directly,  or,  during 
the  intervals  of  digestion,  to  the  gall  bladder.  When  food 
enters  the  duodenum,  a  reflex  influence  causes  the  wall  of  the 
gall  bladder  to  contract  and  compress  its  contents.  The  only 
outlet  is  through  the  cystic  duct  into  the  common  duct,  thence 
into  the  duodenum.  This  reflex  does  not  take  place  until 
food  has  entered  the  duodenum,  and  of  different  foods  it  is 
found  that  proteids  (peptones)  and  fats  are  the  most  effi- 
cient stimuli. 

The  secretion  of  bile  is  not  stopped  by  ligation  of  either 
the  portal  vein  or  the  hepatic  artery,  showing  that  both  of 
these  vessels  contain  bile  materials.  But  it  would  be  unrea- 
sonable to  suppose  that  the  blood  of  the  portal  vein  does  not 
furnish  the  bulk  of  secreting  material. 

Glycogenic  Function. — The  formation  of  glycogen  is  con- 
nected with  nutrition,  but  will  receive  some  notice  here. 
This  is  an  internal  secretion.  It  is  produced  by  the  liver 
cells,  and  can  be  demonstrated  in  their  substance  by  the  mi- 
croscope and  by  chemical  reagents.  It  can  also  be  shown  to 
increase  markedly  after  eating,  and  to  decrease  notably  when 
eating  is  refrained  from  for  some  time. 


THE  LIVER  113 

Glycogen  is  a  carbohydrate  very  similar  to  starch,  and 
when  ingested  it  is  acted  upon  by  the  same  enzymes  and  un- 
dergoes the  same  conversions.  Furthermore,  the  amount  of 
glycogen  in  the  liver  is  very  greatly  increased  by  restricting 
the  diet  to  carbohydrate  foods  and  is  lessened  considerably 
below  the  normal  (that  is,  its  amount  on  a  mixed  diet),  but 
is  not  reduced  to  zero,  when  proteids  alone  are  taken.  This 
points  to  the  conclusion  that  the  source  of  glycogen  is  car- 
bohydrates, but  that  it  can  be  formed  to  some  extent  from 
proteids.  Let  it  be  said  now  that  practically  all  carbohy- 
drates are  converted  by  digestion  into  maltose,  or  maltose 
and  dextrin  and  furthermore  that  during  absorption  these 
sugars  are  converted  into  dextrose  or  dextrose  and  levulose. 
It  is  customary  to  assume  that  the  digestion  of  a  carbohy- 
drate means  its  conversion  into  dextrose  (glucose,  levulose). 
It  is,  then,  this  sugar  which  is  carried  to  the  liver  by  the 
portal  vein. 

We  may  say  that  the  formula  for  dextrose  is  CeH^Oe  and 
for  glycogen  CeHioCte,  though  neither  of  these  formulae  is 
probably  exactly  correct.  It  will  be  seen,  therefore,  that  the 
abstraction  of  one  molecule  of  water  (HsO)  from  dextrose 
will  produce  glycogen,  and  this  is  the  change  which  the  liver 
cells  are  supposed  to  effect.  Again,  when  the  conversion  of 
dextrose  into  glycogen  has  taken  place,  the  glycogen  is  stored 
up  in  the  liver  cells,  to  be  given  off  continuously  to  the  blood 
only  in  such  quantities  as  the  system  may  demand.  The  liver 
thus  becomes  a  warehouse  for  the  storage  of  all  carbohy- 
drates. 

It  will  be  seen  under  Nutrition  that  the  carbohydrates  fur- 
nish the  chief  material  to  be  burned  up  in  the  body  for  the 
purpose  of  liberating  heat  and  furnishing  energy,  and  if  they 
should  be  consumed  as  soon  as  they  enter  the  circulation, 
there  would  be  not  only  an  unnecessary  waste  during  their 
quick  consumption,  but  also  an  unfortunate  lack  of  energy- 
producing  materials  before  another  meal.  This  storing  up 
brings  about  a  kind  of  conservation  of  energy  and  an  eco- 
8 


1 14        THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

nomical  regulation  of  its  distribution.  The  amount  of  sugar 
in  the  circulation  at  any  time  is  very  small,  and  a  single  car- 
bohydrate meal  may,  by  the  action  of  the  liver  be  made  to 
supply  the  carbohydrate  demands  of  the  tissues  for  a  consid- 
erable period. 

Now,  it  was  just  said  that  the  sugar  of  the  blood  is  dex- 
trose; if  the  dextrose  of  the  portal  blood  is  converted  into 
glycogen  to  be  stored  up,  it  must  be  reconverted  into  dex- 
trose before  it  can  leave  the  liver,  since  it  leaves  by  the  blood. 
The  cells  do  effect  the  second  conversion,  and  this  is  the  sec- 
ond part  of  the  glycogenic  function.  It  may  be  that  the  liver 
cells  produce  an  enzyme  corresponding  to  ptyalin,  which  con- 
verts the  glycogen.  Dextrose  does  not  normally  exist  in  the 
liver  cells.  At  the  very  moment  of  its  formation  it  is  car- 
ried away  by  the  blood. 

The  fact  that  the  liver  can  form  glycogen  out  of  pro- 
teids  shows,  of  course,  that  nitrogen  is  eliminated  from  the 
proteid  molecule  in  some  way.  A  carbohydrate  molecule  is 
left  to  be  oxidized  in  the  usual  manner.  This  is  thought  to 
be  the  initial  step  in  the  final  consumption  of  proteids  in  nu- 
trition. The  fats  have  no  influence  on  glycogen  formation. 

Glycogen  also  exists  in  other  parts  of  the  body,  particu- 
larly in  the  voluntary  muscular  substance.  The  cells  of  the 
tissue  in  which  it  is  found  must  also  have  a  glycogenic  func- 
tion. 

Urea  Formation. — But  the  liver  has  another  function  be- 
sides the  production  of  bile  and  glycogen,  and  that  is  to  form 
urea.  It  will  be  seen  later  that  the  chief  end  product  of  pro- 
teid metabolism  is  urea,  and  that  it  is  eliminated  almost  en- 
tirely by  the  kidneys.  The  liver  is  much  more  active  in  the 
production  of  thfs  substance  when  the  portal  blood  is  charged 
with  digested  materials,  but  it  also  forms  urea  in  fasting 
animals.  The  liver  must,  therefore,  be  capable  of  forming 
urea  from  some  of  the  products  of  digested  foods.  With 
reference  to  its  formation  in  fasting  animals,  suffice  it  to  say 
here  that  it  seems  that  as  long  as  proteid  metabolism  goes 


THE  INFLUENCE  OF  THE  BILE  ON  DIGESTION  115 

on  in  other  tissues,  there  are  produced  in  those  tissues  ma- 
terials (ammonia  compounds)  which,  when  carried  to  the 
liver,  are  converted  by  it  into  urea.  Further  notice  will  be 
given  to  this  phase  of  the  subject  under  Nutrition. 

The  liver  cells  produce  urea ;  it  enters  the  blood,  is  carried 
to  the  kidneys  and  eliminated  by  those  organs.  In  the  me- 
chanism of  its  production  and  discharge  from  the  liver,  it 
thus  corresponds  to  the  internal  secretions,  though  urea  is 
distinctly  an  excretion. 

It  must  not  be  supposed,  however,  that  the  liver  is  the  only 
organ  producing  urea.  There  are  other  organs  which  cer- 
tainly produce  it,  while  there  are  those  who  maintain  that  it 
is  produced  directly  wherever  proteid  metabolism  is  in  pro- 
gress. 

The  Influence  of  the  Bile  on  Digestion. 

The  bile  is  not,  properly  speaking,  a  digestive  fluid,  for 
it  contains  no  enzyme  capable  of  effecting  digestive  changes 
in  any  of  the  foods ;  but  it  so  materially  affects  the 
action  of  some  of  the  other  fluids  that  it  cannot  be  overlooked 
in  a  discussion  of  intestinal  digestion. 

So  far  as  the  bile  acids,  glycocholic  and  taurocholic  (com- 
bined to  form  salts  of  sodium)  are  concerned,  the  fluid  is  a 
secretion,  and  it  is  these  which  are  mainly  concerned  in  the 
digestive  process.  The  production  of  bile  is  continuous,  but 
the  gall  bladder  acts  as  a  reservoir  in  which  a  part  at  least  of 
the  secretion  is  stored  in  the  intervals  of  digestion,  to  be  dis- 
charged in  greater  abundance  when  chyme  enters  the  duo- 
denum. While  the  action  of  bile  in  most  of  the  digestive 
functions  to  be  mentioned  is  obscure,  it  is  known  to  have  at 
least  these  uses : 

1.  It  promotes  intestinal  peristalsis.    ' 

2.  It  has  an  inhibitory  effect  on  putrefaction  in  the  intesti- 
nal tract.    By  this  it  is  not  to  be  understood  that  the  bile  is 
directly  antiseptic,  for  it  undergoes  putrefaction  very  read- 
ily itself,  but  only  that  in  some  way  its  withdrawal  from  the 


Il6        THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

substances  passing  through  the  alimentary  canal  allows  their 
more  ready  disintegration. 

3.  It  aids  in  the  emulsification  of  fats. 

4.  It  promotes  the  absorption  of  fats.    Recently  the  state- 
ment that  the  bile  promotes  all  kinds  of  absorption  has  appar- 
ently been  successfully  disproved,  but  it  seems  certain  that 
"the  bile  acids  enable  the  bile  to  hold  in  solution  a  consider- 
able quantity  of  fatty  acids,  and  possibly  this  fact  explains 
its  connection  with  fat  absorption."    (American  Text-Book.) 

The  Secretion  of  the  Intestines. 

The  intestinal  secretion,  or  succus  entericus,  is  a  product 
of  the  crypts  of  Lieberkuhn  and  Brunner's  glands.  It  is 
scanty,  of  a  yellow  color  and  an  alkaline  reaction.  Opinions 
vary  as  to  what  foods  are  affected  by  this  fluid,  but  since  the 
more  recent  experiments  have  overcome  some  difficulties  in 
obtaining  specimens,  the  conclusions  based  upon  them  seem 
most  reliable.  It  is  said  to  have  no  effect  on  proteids  or 
fats.  It  contains  an  amylolitic  enzyme,  which  aids  the  pan- 
creatic juice  in  converting  starch  into  maltose.  It  also  has 
an  enzyme,  invertase,  which  converts  cane  sugar  'into  dex- 
trose and  levulose,  as  well  as  an  allied  enzyme,  maltose, 
which  converts  maltose  into  dextrose.  The  carbohydrates 
are  absorbed  as  dextrose,  with  the  probable  exception  of 
lactose.  It  is  mainly  cane  sugar,  maltose  (from  starch) 
and  lactose  that  are  in  the  alimentary  tract  and  require  to  be 
thus  changed  to  dextrose. 

It  is  not  out  of  place  to  say  that  ptyalin  produces  maltose 
and  a  little  dextrose,  and  that  the  pancreatic  juice  and  succus 
entericus  produce  maltose  and  considerable  dextrose.  The 
maltose  is  converted  into  dextrose  during  the  process  of 
absorption.  It  is,  therefore,  customary  to  say  that  the  carbo- 
hydrates are  absorbed  only  as  dextrose. 

Movements  of  the  Small  Intestine. — The  effect  of  intesti- 
nal movements  is  to  force  the  contents  onward  through  the 


LARGE   INTESTINE  117 

ileocecal  valve.  Here  it  is  that  typical  peristalsis  is  found. 
The  main  factor  in  the  passage  is  the  layer  of  circular  fibers. 
Contraction  of  these  fibers  in  the  upper  duodenum  may  at 
least  be  conceived  to  begin  upon  the  introduction  of  chyme. 
The  contraction  passes  down  the  gut  in  a  wave-like  manner, 
the  wave  being  produced  by  the  contraction  of  segment  after 
segment  of  the  circular  fibers  with  relaxation  just  behind  the 
advancing  contraction.  The  tendency  of  such  a  movement  is 
to  force  the  alimentary  mass  along  the  canal.  The  longitu- 
dinal fibers  are  probably  chiefly  concerned  in  changing  the 
position  of  the  intestine  and  in  shortening  the  tube,  and  thus 
slipping  the  mucous  membrane  above  the  bolus,  so  that  it 
can  be  grasped  by  the  circular  fibers.  A  continuation  and 
repetition  of  these  movements,  which  are  slow,  gentle  and 
gradual  in  character,  is  finally  effectual  in  passing  the  con- 
tents into  the  colon.  It  is  not  probable  that  antiperistaltic 
movements  take  place  normally. 

Nerve  Supply. — Very  probably  the  intestinal  movements 
are  naturally  excited  by  the  food  and  by  the  bile.  It  is  prob- 
able also  that  these  stimuli  exert  their  influence  through  the 
ganglia  of  the  plexuses  of  Auerbach  and  Meissner.  The  in- 
testine receives  fibers  from  the  right  vagus  and  the  sympa- 
thetic. The  former  are  probably  motor  (contractors)  and 
the  latter  inhibitory  (dilators).  Here,  as  in  the  stomach, 
they  are  probably  only  regulators  of  the  movements,  without 
being  actually  necessary  to  peristalsis. 

The  Large  Intestine. 

Anatomy. — The  Jarge  intestine,  known  as  the  colon,  is 
about  five  feet  in  length  and  is  divided  into  ascending,  trans- 
verse and  descending  portions.  The  sigmoid  flexure  is  the 
terminal  extremity  of  the  descending  colon  and  empties  into 
the  rectum.  The  small  intestine  communicates  with  the 
colon  at  right  angles  a  little  above  the  beginning  of  the  latter, 
leaving  below  the  opening  a  blind  pouch,  the  cecum,  or 


Il8        THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

caput  coli.  From  the  posterior  and  inner  aspect  of  the  cecum 
runs  off  the  appendix  vermiformis.  The  diameter  of  the 
colon  gradually  decreases  from  two  and  a  half  to  three  and 
a  half  inches  in  the  cecum  to  the  beginning  of  the  rectum. 
The  ascending  colon  passes  upward  from  its  beginning  in  the 
right  iliac  fossa  to  the  under  surface  of  the  liver,  where  it 
bends  upon  itself  almost  at  a  right  angle  (hepatic  flexure). 
The  transverse  colon  runs  directly  across  the  upper  part  of 
the  abdominal  cavity  to  the  lower  border  of  the  spleen,  where 
an  abrupt  turn  downward  (splenic  flexure)  begins  the  de- 
scending colon.  The  lower  part  of  the  descending  colon  oc- 
cupies the  left  iliac  fossa  in  the  shape  of  the  letter  S,  and  is 
the  sigmoid  flexure. 

The  rectum,  which  receives  the  contents  of  the  sigmoid,  is 
not  straight,  as  its  name  indicates.  It  curves  ( i )  to  the  right 
to  reach  the  median  line,  (2)  forward  to  follow  the  contour 
of  the  sacrum,  and  (3)  backward  in  the  last  inch  of  its 
course.  It  has  the  shape  of  a  dilated  pouch,  its  lower  ter- 
mination at  the  anus  being  guarded  by  the  powerful  external 
sphincter  of  striated  muscle.  Its  diameter  is  greatest  below. 

The  vermiform  appendix  has  the  three  coats  common  to 
the  intestine,  but  its  muscular  coat  is  ill-developed.  The 
peritoneal  coat  generally  forms  a  short  meso-appendix  at  the 
root  of  the  organ.  The  blood  supply  of  the  organ  is  not 
abundant.  It  is  greater  in  the  female  than  in  the  male,  a 
part  of  it  coming  through  the  appendiculo-ovarian  ligament. 
The  appendix  has  no  function. 

The  ileo-cecal  valve,  guarding  the  opening  between  the 
large  and  small  intestines,  is  made  of  two  folds,  upper  and 
lower,  of  the  muscular  and  mucous  coats,  which  folds  pro- 
ject into  the  large  intestine.  The  serous  coat  runs  directly 
over  from  the  small  to  the  large  intestine  at  their  point  of 
junction,  without  being  folded  inward  upon  itself,  as  are 
the  others.  This  prevents  obliteration  of  the  folds  by  dis- 
tention.  By  this  arrangement  the  two  portions  of  the  gut 
communicate  only  by  a  buttonhole  slit,  which  is  easily 


LARGE  INTESTINE  119 

opened  by  pressure  from  the  direction  of  the  ileum  but 
which  pressure  from  the  cecum  tends  to  close  more  firmly. 

Structure. — The  large  intestine  has  the  usual  three  coats. 
The  peritoneal,  however,  is  lacking  on  the  posterior  part  of 
the  cecum,  ascending  and  descending  colons,  these  parts 
being  bound  down  closely  and  having  no  meso-colon.  The 
sigmoid  is  entirely  covered  as  is  the  upper  third  of  the  rec- 
tum. The  middle  third  of  the  rectum  has  no  serous  coat  be- 
hind, being  firmly  held  in  place,  while  the  lower  third  lacks 
this  coat  entirely.  The  muscular  coat  is  peculiar,  in  that  its 
longitudinal  fibers  are  collected  into  three  quite  strong  bands, 
evident  to  the  eye.  When  the  rectum  is  reached  they  spread 
out  over  the  whole  circumference  of  that  part  of  the  canal. 
These  bands  are  shorter,  as  it  were,  than  the  wall  proper,  and 
the  consequence  is  that  the  whole  length  of  the  large  intes- 
tine is  gathered  up  into  a  number  of  pouches.  The  mucous 
coat  is  paler  than  that  of  the  small  intestine,  presents  no  villi 
and  is  rather  closely  adherent  to  the  subjacent  parts.  In  it 
are  found  glands  corresponding  in  appearance  to  the  crypts 
of  Lieberkuhn,  and  they  are  so  classed;  but  they  probably 
secrete  mucus  only.  Some  solitary  lymphoid  follicles  also 
usually  exist  here. 

Changes  Taking  Place  in  the  Alimentary  Mass  in  the 
Large  Intestine. — Most  of  the  substances  which  enter  the 
large  intestine  have  resisted  the  action  of  the  various  diges- 
tive fluids  and  are  on  their  way  to  be  discharged  in  defeca- 
tion. Doubtless,  though,  some  materials  undergo  digestive 
changes  in  the  colon,  not  under  the  influence  of  any  secretion 
there  formed,  but  of  the  intestinal  juice  with  which  they  are 
incorporated  on  leaving  the  ileum.  The  secretion  of  the 
mucous  membrane  of  the  large  intestine  furnishes  no  diges- 
tive enzyme,  and  the  changes  going  on  in  the  alimentary 
mass  (now  feces)  are  chiefly  due  to  absorption.  By  some 
unknown  process,  however,  rectal  aliments  of  an  easily  di- 
gestible nature  are  absorbed,  and  that  in  a  nutritive  form. 
The  consistence  of  the  fecal  matter  increases  in  its  passage 


I2O        THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

through  the  colon,  owing  to  the  absorption  of  its  more  fluid 
portions.  The  bile  pigment  is  responsible  for  the  character- 
istic color.  The  odor  is  mainly  due  to  bacterial  decomposi- 
tion, but  partly  to  the  secretion  of  the  mucous  membrane. 

Bacteria  in  Intestinal  Digestion. — The  entrance  of  the 
bile  and  pancreatic  juice  into  the  duodenum  changes  to  alka- 
line the  previously  acid  reaction  of  the  chyme.  But  it  is 
found  that,  when  an  ordinary  mixed  diet  is  given,  the  mass 
leaving  the  ileo-cecal  valve  has  an  acid  (proteid)  reaction, 
and  that  the  proteids  have  not  undergone  putrefaction.  The 
alkaline  medium  of  the  upper  intestine  favors  bacterial  ac- 
tivity, and  it  would  seem  that  proteid  putrefaction  would  en- 
sue. But  it  is  supposed  that  in  health  these  bacteria  set  up 
fermentative  changes  in  the  carbohydrates,  with  the  produc- 
tion of  acids  which  inhibit  proteid  putrefaction,  and  account 
for  the  acid  reaction  at  the  ileo-cecal  valve.  When  the  mass 
has  entered  the  colon  the  acidity  is  soon  overcome  and  putre- 
faction is  the  usual  consequence.  It  can  be  seen  how  readily 
this  delicately  adjusted  balance  may  be  disturbed  by  errors  in 
the  proper  kind  and  proportion  of  food,  etc.  Some  of  the 
products  of  bacterial  activity  upon  carbohydrates  and  pro- 
teids are  luecin,  tyrosin,  indol,  skatol,  phenol,  lactic  and  bu- 
tyric acid.  The  object  of  the  production  of  these  substances 
is  unknown. 

Composition  of  Feces. — It  seems  at  present  that  the  main 
bulk  of  fecal  matter  is  made  up  of  substances  which  are  con- 
tained in  the  intestinal  secretions,  and  the  alimentary  canal  is 
more  important  in  excretion  than  was  formerly  supposed. 
These  substances  are  waste  matters  from  tissue  metabolism. 
Besides  these  materials,  feces  normally  contain  indigestible 
and  undigested  matters,  inactive  salts,  stercorin,  mucus,  epi- 
thelium from  the  intestinal  wall,  coloring  matter  and  sub- 
stances resulting  from  bacterial  activity.  Stercorin  is  the 
converted  form  of  cholesterin,  a  constituent  of  the  bile.  The 
coloring  matter  is  from  the  pigment  (bilirubin)  of  the  same 
fluid.  Of  the  bacterial  products  the  most  important  are  in- 


LARGE  INTESTINE  121 

dol  and  skatol.  They  represent  proteid  putrefaction;  they 
are  responsible  for  the  fecal  odor;  hence  the  characteristic 
difference  in  the  odor  of  the  contents  of  the  ileum  and  colon. 
The  reaction  of  fecal  matter  varies.  The  amount  for  the  av- 
erage person  is  about  four  and  a  half  ounces  per  day. 

Gases. — Hydrogen,  nitrogen  and  cafbon  dioxide  are  found 
normally  in  the  small  intestines.  They  serve  to  keep  the  tube 
patulous,  and  avoid  obstruction,  and  also  to  prevent  con- 
cussion. In  the  large  intestine  bacterial  activity  increases  the 
number  of  gases  present.  Here,  in  addition  to  those  found 
in  the  small  intestine,  there  are  carbitretted  and  sulphuretted 
hydrogen,  with  others  at  times. 

Movements  of  the  Large  Intestine. — The  muscular  con- 
tractions of  the  colon  forcing  the  feces  onward  are  of  the 
same  general  character  as  those  of  the  small  intestine,  though 
less  violent.  The  contents  thus  passed  analward  by  peristal- 
sis accumulate  gradually  in  the  sigmoid  flexure  until  defeca- 
tion occurs. 

Defecation. — The  act  of  defecation  is  both  voluntary  and 
involuntary — voluntary  in  the  relaxation  of  the  external 
sphincter  and  involuntary  in  the  peristalsis  which  brings  the 
fecal  matter  to  present  at  that  muscle.  It  is  probable  that  the 
rectal  pouch  does  not  usually  contain  feces,  but  the  desire  to 
defecate  is  brought  about  by  the  entrance  of  the  mass  into  it 
from  the  sigmoid.  Then,  if  the  desire  is  obeyed,  peristalsis 
of  the  non-striated  muscular  coat  continues,  the  internal 
sphincter  of  plain  muscle  relaxes,  as  does  also  the  external 
of  striped  muscle,  and  evacuation  takes  place. 

Usually,  by  an  effort  of  will,  evacuation  can  be  voluntarily 
prevented  by  maintaining  the  tonic  contraction  o'f  the  exter- 
nal sphincter.  If  the  desire  to  defecate  be  disregarded,  the 
fecal  accumulation  probably  returns  to  the  sigmoid,  leaving 
the  rectum  comparatively  empty.  The  act  of  evacuation  is 
commonly  aided  further  by  voluntary  contraction  of  the 
diaphragm  and  abdominal  muscles.  The  lungs  are  filled, 
"the  breath  is  held"  (forcing  down  and  holding  the  dia- 


122        THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

phragm),  and  the  abdominal  muscles  likewise  contract 
powerfully  to  compress  the  viscera  and  force  the  feces  into 
the  rectum.  Pressure  on  the  afferent  nerves  of  the  rectum 
probably  sets  up  the  desire  to  defecate,  and  the  contraction  of 
its  walls,  as  well  as  the  relaxation  of  the  internal  sphincter  is 
a  reflex  act.  The  center  is  in  the  lower  segment  of  the  cord, 
but  it  is  connected  with  the  cerebrum,  as  is  shown  by  emo- 
tional influences  on  the  act. 

The  average  time  occupied  in  the  passage  of  the  residue 
of  an  ordinary  meal  from  the  mouth  to  the  rectum  is  about 
24  hours.  Something  like  12  hours  of  this  is  thought  to  be 
spent  in  the  large  intestine. 

While  it  has  been  endeavored  to  establish  clearly  the  sep- 
arate action  of  each  fluid  with  which  the  aliment  comes  in 
contact,  it  is  to  be  remembered  that  they  form  a  mixture,  the 
combined  activity  of  whose  component  parts  results  in  the 
extraction  of  all  the  nutritive  material  from  the  bolus  in  its 
long  journey  through  the  gastro-intestinal  tract.  It  can 
hardly  be  said  to  be  still  at  any  time  during  that  passage, 
the  continual  peristalsis  to  which  it  is  subjected  facilitating 
both  the  chemical  action  of  the  enzymes  and  the  physical 
phenomenon  of  absorption. 

ABSORPTION  IN  GENERAL. 

Obviously  digested  materials  are  of  no  service  in  the  vital 
economy  until  they  are  absorbed — first  by  the  circulation  and 
then  by  the  tissues  themselves.  Here  we  will  consider  only 
their  absorption  from  the  alimentary  canal,  which  process,  in 
contradistinction  to  the  other,  may  be  termed  external  ab- 
sorption. 

While  it  is  known  that  the  laws  of  diffusion  and  osmosis 
outside  the  body  are  largely  responsible  for  absorption  within 
the  organism  there  are  many  phenomena  in  connection  with 
that  process  which  cannot  be  explained  under  these  laws, 
and  which  are  indeed,  in  some  cases,  at  variance  with  them. 


ABSORPTION    IN   GENERAL  123 

The  only  explanation  at  present  to  be  offered  of  anomalous 
action  is  to  refer  it  to  some  peculiar  property  inherent  in  the 
cells  themselves — the  epithelium  in  case  of  the  alimentary 
canal.  So  profoundly  important  in  connnection  with  physio- 
logical activity  are  the  laws  of  osmosis  outside  of  the  body, 
and  what  is  known  concerning  the  mutability  of  those  laws 
inside  the  body,  that  a  brief  consideration  of  the  subject 
seems  necessary  to  an  intelligent  conception  of  many  vital 
phenomena. 

Osmosis. — When  two  different  kinds  of  gases  are  brought 
in  contact  they  mingle  with  each  other,  making  a  homogen- 
eous mixture.  This  is  due  to  the  continual  motion  of  their 
molecules.  When  two  different  kinds  of  liquids  are  brought 
in  contact,  a  homogeneous  mixture  results  for  the  same  rea- 
son— unless  the  liquids  be  non-miscible,  as  oil  and  water. 
If  now  the  liquids  happen  to  be  separated  by  a  membrane 
permeable  by  both,  the  result,  while  it  may  be  delayed,  will 
be  the  same.  If,  further,  these  liquids  hold  in  solution  sub- 
stances the  molecules  of  which  can  penetrate  the  interposed 
membrane,  there  will  likewise  be  an  interchange  of  these  sub- 
stances, and  the  fluids  on  both  sides  will  come  ultimately  to 
have  the  same  composition.  This  passage  of  liquids  and  dis- 
solved matters  through  an  animal  membrane  is  known  as 
osmosis. 

It  must  be  remembered  that  in  the  body  particularly  the 
interposed  membrane  may  be  permeable  to  the  solvent, 
water,  and  less  so,  or  not  at  all,  to  the  dissolved  substances. 
Materials  which  will  in  solution  pass  through  a  membrane 
are  called  crystalloids;  those  which  will  not,  colloids.  If 
simple  water  be  on  both  sides  of  the  membrane,  the  inter- 
change continues  because  of  incessant  molecular  motion ;  but 
the  currents  equalize  each  other,  and  no  alteration  in  volume 
or  composition  becomes  apparent.  But  if  to  the  water  on 
one  side  there  be  added  a  solution  of  some  crystalloid,  as 
sugar,  the  excess  of  water  will  pass  to  that  side.  The  crys- 
talloid in  solution  is  said  to  exert  an  osmotic  pressure,  and 


124        THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

that  pressure  depends  upon  the  density  of  the  solution.  In 
course  of  time,  however,  the  crystalloid  passing  itself  through 
the  membrane,  conditions  of  equal  volume  and  density  will 
be  established  on  the  two  sides  of  the  membrane,  and  osmosis 
in  either  direction  will  cease  to  be  apparent.  But  if  the 
membrane  be  nonpermeable  to  the  dissolved  substance,  an 
excess  of  water  will  pass  to  the  colloid  side  and  will  continue 
so  to  pass  until  finally  it  will  be  inhibited  by  hydrostatic 
pressure  on  that  side.  This  is  taken  as  the  measure  of  os- 
motic pressure  for  the  colloid. 

All  substances  in  solution,  whether  crystalloids  or  colloids, 
exert  a  certain  osmotic  pressure ;  that  is,  they  may  be  said  to 
interfere  with  the  passage  of  a  current  from  their  side  of  the 
membrane,  and  that  interference  depends  on  the  number  of 
molecules  in  solution,  or,  in  other  words,  upon  the  density  of 
the  fluid.  A  fanciful  but  striking  illustration  refers  the  ex- 
planation to  the  continual  molecular  motion:  the  molecules 
of  the  dissolved  substance  act  as  a  screen  to  protect  the 
membrane  from  the  water  molecules,  which  are  incessantly 
moving  against  it,  and  consequently,  in  a  given  time,  more 
molecules  of  water  will  strike  and  pass  through  the  mem- 
brane on  the  unscreened  than  upon  the  partially  screened 
side.  Evidently  the  number  of  molecules  in  solution  (the 
density)  has  a  material  influence  upon  the  escape  of  water 
from  that  side.  Of  course,  since  a  crystalloid  finally  passes  to 
.  the  less  dense  side  in  sufficient  quantity  to  establish  an  equi- 
librium, the  effect  of  its  osmotic  pressure  is  only  temporary ; 
but  while  the  osmotic  pressure  of  a  colloid  may  be  less  than 
that  of  a  crystalloid,  its  effect  is  inclined  to  be  permanent. 
For  instance,  if  a  hypertonic  solution  (one  whose  density  is 
greater  than  that  of  blood  serum)  of  sodium  chloride  be  in- 
jected into  the  blood,  the  first  effect  is  to  cause  an  increased 
flow  of  water  to  the  vessels,  but  soon  enough  sodium  chloride 
passes  out  by  osmosis  to  raise  the  density  of  the  extravascu- 
lar  fluids,  and  thus  to  cause  an  escape  of  water  from  the  ves- 
sels. On  the  other  hand,  the  osmotic  pressure  exerted  by  the 


ABSORPTION  IN  GENERAL  125 

proteids  of  the  blood  is  comparatively  small.  But  since  they 
are  here  chiefly  as  colloids  and  tend  to  maintain  the  concen- 
tration of  the  circulating  fluid,  their  effect  is  a  permanent 
factor  influencing  absorption  into  the  blood-vessels. 

Isotonic  and  hypotonic  solutions  are  those  having  equal  and 
less  densities  respectively  as  compared  to  blood  serum.  Hy- 
potonic solutions  are  most  easily  absorbed ;  isotonic  least 
easily.  Application  of  these  principles  explains  the  rationale 
of  giving  some  medicines  in  dilute  and  others  in  concentrated 
form.  As  to  the  direction  of  the  current,  the  one  of  greater 
volume  may  be  called  the  endosmotic  and  the  one  of  lesser 
volume  may  be  called  ex  osmotic.  For  example,  the  current 
in  ordinary  absorption  from  the  alimentary  canal  is  usually 
termed  endosmotic,  though  it  may  be  reversed,  as  when 
magnesium  sulphate  is  given. 

When  it  is  said  that  the  greater  current  is  from  the  less 
dense  to  the  more  dense  fluid,  no  reference  is  had  to  the  di- 
rection of  the  solids  in  solution.  If  there  be  only  one  solid 
concerned,  it  will  be  the  one  responsible  for  the  difference  in 
density  and  if  it  be  a  crystalloid,  it  will  pass  through  the 
membrane  until  the  density  on  the  two  sides  is  equal,  and  its 
direction  will  be  opposite  to  that  of  the  water.  If  on  the 
side  of  less  density  there  be  another  crystalloid  in  solu- 
tion, but  in  less  quantity  than  the  solid  on  the  side  of  greater 
density,  it  will  pass  in  the  direction  of  the  greater  current  of 
water  until  conditions  of  equal  concentration  with  respect 
to  this  solid  are  established.  In  the  laboratory  the  final  re- 
sult in  any  case  of  dissolved  crystalloid  or  crystalloids  is  two 
liquids  absolutely  identical  in  composition.  A  rectal  enema, 
hypertonic  with  sodium  chloride,  will  give  up  sodium  chlor- 
ide to  the  blood,  but  it  may  at  the  same  time  draw  upon  that 
fluid  for  urea,  for  example.  This  is  suggestive  when  an  at- 
tempt is  made  to  explain  the  products  of  glandular  secretion, 
excretion,  etc.  It  may  be  that  the  capillary  walls  are  per- 
meable to  certain  substances  in  certain  situations  and  not  in 
others. 


126        THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

In  the  body  it  may  be  said  that  well-nigh  all  the  vital  func- 
tions are  dependent  upon  osmosis.  There  are  fluids  separ- 
ated by  animal  membranes  everywhere.  In  the  alimentary 
canal,  for  instance,  is  a  fluid  containing  matters  fit  to  be  ab- 
sorbed; ramifying  in  the  wall  of  that  canal  are  blood  and 
lymph  capillaries  filled  with  fluid ;  while  separating  the  two 
is  an  animal  membrane  consisting  of  the  alimentary  epithe- 
lium, a  little  connective  tissue  and  the  endothelial  lining  of 
the  capillaries.  These  are  conditions  most  favorable  for  os- 
mosis, but  the  osmotic  laws  of  the  laboratory  are  by  no  means 
immutable  in  the  body. 

From  what  has  been  said  of  osmosis  in  general,  and  con- 
sidering variations  due  to  conditions  of  circulation,  etc.,  the 
following  facts  seem  clear  as  to  absorption  in  the  body :  ( i ) 
The  substance  must  be  in  a  liquid  or  gaseous  state;  (2)  it 
must  be  diffusible;  (3)  the  membrane  must  be  permeable; 

(4)  the  greater  current  is  toward  the  more  dense  solution 

(5)  the  less  dense  the  solution  the  more  quickly  will  it  be  ab- 
sorbed; (6)  the  greater  the  pressure  in  the  vessels  the  less 
rapidly  will  absorption  into  them  take  place;  (7)  absorption 
is  more  rapid  the  more  rapid  the  blood  current  (continually 
preventing  "saturation"  of   the  adjacent  blood)  ;    (8)    the 
higher  the  temperature  the  more  rapid  is  absorption;   (9) 
the  "vital  condition"  of  the  cells  is  the  most  important  fac- 
tor of  all. 

A  thorough  grasp  of  these  principles  and  probabilities  will 
do  much  to  clarify  almost  all  the  phenomena  of  vital  activity, 
and  many  questions  of  a  pathological  nature. 

Absorption  from  the  Alimentary  Canal. 

It  has  been  said  that  all  digested  materials  must  find  their 
way  into  the  blood.  It  is  to  be  remembered  that  there  are 
two  ways  by  which  they  reach  the  vascular  circulation ;  first, 
by  direct  absorption  into  the  capillaries  of  this  system,  and 
second,  indirectly,  by  absorption  into  the  lymphatic  circula- 


ABSORPTION    FROM    THE   ALIMENTARY    CANAL  1 27 

tion  and  passage  thence  to  the  left  subclavian  vein.  Those 
lymph  capillaries  which  are  concerned  in  this  absorption  oc- 
cupy the  villi,  and  are  called  lacteals. 

(A)  From  the  Stomach. — Since  all  classes  of  food  except 
fats  have  been  partly  digested  in  the  stomach,  it  follows  that 
all  except  fats  may  be  absorbed  here.    However,  as  a  mat- 
ter of  observation,  the  stomach  is  of  much  less  importance 
in  absorption  than  was  once  thought.    Practically,  it  is  found 
that  water  and  salts  are  passed  quickly  on  toward  the  duo- 
denum and  are  not  largely  absorbed  in  the  stomach.     Sugar 
and  peptones  are  also  found  to  be  absorbed  rather  sparingly 
here.    All  these  substances  can  undoubtedly  be  absorbed  by 
the  gastric  mucous  membrane,  and  their  complete  absorption 
is  prevented  only  by  their  removal  through  the  pylorus.     It 
is  interesting  to  note  that  alcohol  and  condiments,  like  pep- 
per and  mustard,  greatly  hasten  absorption,  either  by  in- 
creasing the  blood  flow  or  by  directly  stimulating  the  "vital 
activity"  of  the  epithelium. 

(B)  From  the  'Small  Intestine. — Here  absorption  of  all 
classes  of  food  is  possible,  and  here  in  fact  most  of  the  foods 
are  absorbed.    The  digestive  influences  are  more  active  upon 
all  the  aliments,  the  mucous  membrane  is  well  adapted  to 
absorption  by  reason  of  its  valvulae  conniventes  and  its  villi, 
and  the  food  necessarily  remains  in  the  small  intestine  for  a 
considerable  time.       The  fats  are  absorbed  in  the  upper 
part  of  the  small  intestine ;  for  they  pass  into  the  lacteals  of 
the  villi,  and  these  do  not  exist  in  the  lower  ileum.     The 
fluids  swallowed  are  almost  completely  absorbed  here,  but 
their  place  is  taken  by  the  intestinal  secretions.     The  pro- 
teids  are  absorbed  to  the  extent  of  85  per  cent.,  more  or 
less,  before  reaching  the  large  intestine,  and  the  carbohy- 
drates almost  entirely  disappear. 

(C)  From  the  Large  Intestine. — The  absorption  process 
in  the  large  intestine  is  quite  active.      The  passage  of  the 
mass  through  it  is  slower,  and  even  occupies  an  absolutely 
greater  time  than  the  journey  through  the  much  longer  small 


128        THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

intestine.  The  consistence  of  the  contents  progressively  in- 
creases owing  to  continual  absorption  of  the  fluid  portions, 
until  the  pultaceous  mass  received  by  the  cecum  becomes 
almost  solid  in  the  sigmoid.  The  degree  of  consistence  may 
be  said  to  be  greater  the  longer  the  sojourn  in  the  large  in- 
testine. The  proteids  and  carbohydrates  which  have  es- 
caped absorption  in  the  small  intestine  are  disposed  of  here, 
partly  by  bacterial  decomposition,  and  do  not  appear  as  such 
in  the  feces.  The  absorption  of  easily  digestible  substances 
in  solutions,  such  as  eggs,  etc.,  from  the  lower  bowel,  al- 
though there  is  no  digestive  enzyme  there,  is  a  matter  of 
common  observation,  but  one  which  lacks  explanation. 

Forms  in  Which  the  Different  Classes  Are  Absorbed,  i. 
Water  and  Salts. — Of  course,  water  is  absorbed  in  connec- 
tion with  all  the  foods  as  a  vehicle  for  them,  but  water  and 
salts  as  such  have  been  shown  to  be  absorbed  sparingly  in 
the  stomach.  They  are  soon  conveyed  to  the  small  intestine, 
where  their  rapid  disappearance  ensues.  Hbwever,  they 
may  be  absorbed  anywhere  in  the  alimentary  canal.  The  loss 
of  the  water  from  the  alimentary  mass  in  the  upper  small  in- 
testine is  compensated  for  by  the  secretions,  so  that  the  flu- 
idity of  the  contents  is  not  materially  affected  until  the  colon 
is  reached.  Here  absorption  of  water  is  active,  and  the 
mass  becomes  more  and  more  solid  as  the  rectum  is  ap- 
proached. 

2.  Proteids. — It  is  agreed  that  the  first  object  of  proteid 
digestion  is  to  render  the  nitrogenous  foods  more  diffusible. 
It  is  also  agreed  that  the  end  products  of  such  digestion,  so 
far  as  alimentary  absorption  is  concerned,  are  proteoses  and 
peptones;  and  the  natural  conclusion,  supported  by  experi- 
mental evidence,  is  that  these  represent  the  forms  in  which 
the  proteids  are  absorbed.  True,  leucin,  ty rosin,  etc.,  fur- 
ther end  products  of  proteolysis,  are  formed,  but  these  can 
not  be  absorbed.  The  opinion  that  proteoses  and  peptones 
are  the  absorbable  forms  of  proteids  is  correct,  for  by  far 
the  largest  part  of  these  foods  are  absorbed  in  this  shape. 


ABSORPTION    FROM    THE   ALIMENTARY    CANAL  I2Q 

It  is  supposed  also  that  syntonin  at  least  can  itself  be  spar- 
ingly absorbed  from  the  alimentary  canal,  while  the  phe- 
nomena of  rectal  absorption  would  point  to  the  conclusion 
that  proteid  absorption  in  other  shapes  is  possible.  Prac- 
tically, however,  proteoses  and  peptones  may  be  regarded  as 
the  products  cf  proteid  digestion,  and  their  production  as 
the  object  of  proteolysis. 

But,  although  these  substances  are  absorbed  by  the  blood- 
vessels, the  artificial  injection  of  them  into  the  veins  occa- 
sions untoward  effects,  or  at  least  their  rejection  through 
the  organs  of  excretion.  Furthermore,  proteoses  and  pep- 
tones cannot  be  detected  in  the  blood  during  alimentary  ab- 
sorption. It  follows,  then,  that  in  their  passage  from  the 
alimentary  canal  to  the  blood  they  undergo  some  change 
whereby  they  lose  their  identity  and  are  no  longer  recognis- 
able as  such.  It  is  claimed  that  they  are  converted  into 
serum-albumin,  and  this  is  probably  true.  One  effect  at 
least  of  the  change  is  that  they  are  now  (in  the  blood)  less 
diffusible,  more  complex,  and  consequently  remain  more 
easily  a  constituent  part  of  that  fluid. 

The  proteids  enter  the  radicles  of  the  portal  vein. 

3.  Carbohydrates. — The  sugar  of  the  blood  is  dextrose, 
and  if  cane  sugar  be  introduced  into  the -veins  it  is  rejected 
by  the  urine  without  being  changed.     It  may  be  said  that, 
with  a  few  exceptions,  all  the  carbohydrates  are  converted 
into  dextrose  or  dextrose  and  levulose,  before  entering  the 
blood.    This  form  of  sugar  is  easily  oxidized  in  the  tissues. 
It  is  conveyed  directly  to  the  liver  by  the  portal  vein. 

4.  Fats. — The  digestive  end  of  the  fats  has  been  seen  to 
be  emulsions  and  soaps. .  They  pass  into  the  intestinal  lym- 
phatics, or  lacteals.     Their  absorption  is  a  mechanical  pro- 
cess.    They  enter  and  pass  through  the  epithelial  cells  and 
basement  membrane  of  the  villus.    Having  thus  passed  into 
the  stroma  of  the  villus,  their  entrance  into  the  lacteal  is 
easy ;  for  undoubtedly  lymph  spaces  in  the  stroma  .are  con- 
nected with  the  stomata  of  the  central  lymph  capillary,  and 

-9 


130        THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 

there  is  a  more  or  less  constant  flow  of  lymph  through  these 
spaces  toward  the  lacteal.  The  tendency,  therefore,  of  the 
fats  to  enter  the  lacteal  is  physically  natural.  It  is  a  curious 
fact  that  the  peptones  and  sugars,  having  penetrated  the  lin- 
ing epithelium  of  the  villus,  enter  the  blood  instead  of  the 
lymph  capillaries. 

A  number  of  circumstances,  such  as  the  rate  of  absorp- 
tion, the  persistent  direction  of  the  current  toward  the  blood 
in  the  face  of  superior  pressure,  the  disappearance  of  non- 
osmotic  substances  from  the  canal,  etc.,  are  frequently  at 
variance  with  laboratory  experiments.  Application  of 
the  laws  of  osmosis  to  the  vital  processes  is  seemingly  sub- 
ject to  many  variations,  and  explanation  of  many  of  the  phe- 
nomena of  absorption  in  the  body  waits  upon  a  clearer  un- 
derstanding of  the  so-called  "vital  activity"  of  the  tissues. 


CHAPTER  VIII. 
RESPIRATION. 

Object. — The  object  of  respiration  is  to  furnish  oxygen  to 
the  tissues  and  remove  carbon  dioxide  from  them.  The  in- 
tervention of  the  lungs  and  blood  is  necessary  to  accom- 
plish this  end.  At  each  inspiration  a  certain  volume  of  air 
is  taken  into  the  lungs,  and  from  it,  while  in  these  organs,  is 
removed  a  certain  amount  of  oxygen  which  enters  the  blood 
of  the  pulmonary  capillaries.  At  each  expiration  there  is  re- 
moved from  the  lungs  a  certain  volume  of  air,  and  it  con- 
tains a  proportion  of  carbon  dioxide  over  and  above  that 
contained  in  the  ordinary  atmosphere,  i.  e.,  in  the  inspired 
air ;  this  carbon  dioxide  is  removed  from  the  blood  of  the  pul- 
monary capillaries  and  enters  the  air  in  the  lungs.  The  en- 
trance and  exit  of  air  to  and  from  the  lungs,  in  obedience  to 
movements  to  be  noticed  later,  constitutes  what  is  commonly 
called  respiration;  but  the  mere  tide  of  the  air  inward  and 
outward  is  of  no  significance  unless  the  interchange  of  oxy- 
gen and  carbon  dioxide  takes  place. 

Internal  Respiration. — Nor  is  this  interchange  of  value 
unless  another  occurs  in  the  tissues.  The  oxygen  which  has 
entered  the  pulmonary  blood  is  conveyed  by  the  circulation 
to  a  point  where  the  fluid  is  brought  into  very  close  relation- 
ship with  the  tissues  (namely,  in  the  capillaries),  and  is  here 
given  up  to  the  cells;  furthermore,  at. the  same  place  the 
cells  give  up  carbon  dioxide  to  the  capillary  blood.  It  is  only 
for  the  purpose  of  effecting  this  last  interchange  that  there 
is  any  respiration,  or  any  respiratory  apparatus.  Inspira- 
tion and  expiration,  the  pulmonary  interchange  of  gases,  the 
transportation  of  oxygen  and  carbon  dioxide  to  and  away 
from  the. cells,  are  all  equally  immaterial  except  as  being 
means  to  the  accomplishment  of  this  end.  It  would  make  no 
difference  whether  pulmonary  respiration  were  kept  up  or 


132  RESPIRATION 

not  if  oxygen  could  be  introduced  into  the  blood  and  carbon 
dioxide  removed  from  it  in  some  other  equally  efficient  way. 
So  far  as  the  cell  is  dependent  on  the  acquisition  of  oxygen 
and  the  removal  of  carbon  dioxide,  it  would  make  no  differ- 
ence if  there  were  no  respiration  and  no  circulation  if  these 
materials  could  be  acquired  and  removed  in  some  other 
equally  efficient  way. 

On  the  other  hand,  it  would  be  useless  to  keep  up  artificial 
respiration  or  to  inject  oxygen  into  the  lungs  if  the  cells, 
through  some  disability,  cannot  take  up  the  oxygen  fur- 
nished, or  if  the  circulation  cannot  absorb  or  convey  the 
oxygen. 

It  is  seen  that,  from  the  standpoint  of  the  blood,  the  inter- 
change of  gases  on  the  lungs  is  exactly  opposite  to  that  in  the 
tissues ;  that  is  to  say,  in  the  lungs  it  loses  carbon  dioxide  and 
gains  oxygen,  while  in  the  tissues  it  loses  oxygen  and  gains 
carbon  dioxide.  The  pulmonary  interchange  is  properly 
termed  external  respiration  in  contradistinction  to  that  in 
the  tissues  which  is  termed  internal  respiration. 

It  is  needless  to  comment  upon  the  universal  necessity  of 
oxygen  to  the  life  of  cells.  Its  appropriation  is  to  be  looked 
upon  as  a  part  of  the  nutritive  process;  and,  indeed,  while 
in  the  long  run,  cells  are  certainly  dependent  upon  the  nu- 
triment furnished  by  the  ordinary  aliments,  they  will  retain 
their  vital  activity  for  a  longer  time  when  deprived  of  any 
or  all  of  these  than  when  deprived  of  oxygen  alone.  This 
gas  is  more  immediately  necessary  to  the  maintenance  of  life 
than  is  any  other  substance. 

Since,  in  order  to  bring  about  internal  respiration  in  the 
human  being,  the  lungs  and  circulation  happen  to  be  nec- 
essary, attention  will  have  to  be  directed  to  the  respiratory 
phenomena  taking  place  in  both. 

ANATOMY  OF  THE  RESPIRATORY  ORGANS. 

It  will  be  considered  that  the  air  has  passed  through  the 
posterior  nares  into  the  pharynx  and  is  ready  to  enter  the 
larynx. 


ANATOMY  OF  THE  RESPIRATORY  ORGANS 


133 


The  Larynx. — This  lies  in  front  of  the  esophagus,  its 
upper  opening  communicating  with  the  middle  pharynx.  It 
is  composed  of  four  cartilages  and  the  muscles  and  liga- 
ments which  hold  them  together.  The  cartilages  keep  its 
lumen  constantly  open,  while  the  muscles  effect  movements 
concerned  in  deglutition,  respiration  and  phonation.  The 
cartilages  are  the  thyroid,  cricoid  and  two  arytenoids.  The 


The  wind 
bronchi,  whic 


FIG.  47. — Diagram  of  the  respiratory  organs. 

pipe  leading  down  from  the  larynx  is  seen  to  branch  into  two  large 
lich  subdivide  after  they  enter  their  respective  lungs.   (Yeo.) 


two  alae  of  the  thyroid  meet  at  an  acute  angle  in  front  to 
form  the  Adam's  apple.  The  cricoid  is  at  the  lower  end  of 
the  larynx,  completely  surrounding  it.  The  arytenoids  are 
movable  and  rest  upon  the  back  of  the  cricoid.  (Fig.  48.) 

The  vocal  cords,  two  ligamentous  bands  covered  by  a  thin 
layer  of  mucous  membrane,  stretch  antero-posteriorly  across 
the  upper  end  of  the  larynx,  while  the  false  vocal  cords, 
having  nothing  to  do  with  phonation,  and  pinker  in  color,  are 


134' 


RESPIRATION 


FIG.  48. — Outline  showing  the  general  form  of  the  larynx,  trachea, 
and  bronchi,  as  seen  from  behind. 

h,  great  cornu  of  the  hyoid  bone;  *,  superior,  and  t'  the  inferior,  cornu  of 
the  thyroid  cartilage;  e,  epiglottis;  a,  points  to  the  back  of  both  the  arytenoid 
cartilages,  which  are  surmounted  by  the  cornicula;  c,  the  middle  ridge  on  the 
back  of  the  cricoid  cartilage;  tr,  the  posterior  membranous  part  of  the  trachea; 
b,  b',  right  and  left  bronchi.  (Kirkes  after  Allen  Thomson.) 


THE  TRACHEA  135 

above  and  parallel  with  the  true  cords.  A  small  triangular 
leaflet  of  fibre-cartilage  is  attached  by  its  base  to  the  base 
of  the  tongue  and  to  the  upper  anterior  part  of  the  larynx. 
This  is  the  epiglottis.  It  fits  accurately  over  the  opening  of 
the  larynx,  and  during  the  act  qf  deglutition  is  closed  to  pre- 
vent the  entrance  of  food,  saliva,  etc.  Except  during  deglu- 
tition the  epiglottis  is  raised  and  there  is  free  passage  of  air 
into  and  out  of  the  laryngeal  cavity.  The  vocal  cords  are 
fixed  anteriorly  to  a  point  between  the  alse  of  the  thyroid 
and  posteriorly  to  the  movable  arytenoids.  Intrinsic  muscles 
have  the  power  of  so  moving  the  arytenoids  as  to  separate 
and  approximate  the  posterior  attachments  of  the  cords  and 
thus  increase  or  decrease  the  size  of  the  rima  glottidis.  Dur- 
ing inspiration  these  muscles  act  to  separate  the  cords  and 
allow  free  entrance  of  air  into  the  trachea.  When  this  act 
has  ceased  they  relax  and  the  cords  are  passively  approxi- 
mated. The  expiratory  act  separates  the  cords  and  they  af- 
ford no  obstruction  to  the  exit  of  air.  The  inspiratory  act, 
on  the  other  hand,  tends  to  draw  the  cords  together  and  the 
active  intervention  of  the  muscles  is  necessary  to  keep  the 
glottis  open. 

The  Trachea. — The  trachea  succeeds  the  larynx  in  the  re- 
spiratory tract.  It  begins  at  the  cricoid  cartilage  and 
extends  downward  for  about  four  and  a  half  inches  where  it 
bifurcates  to  form  the  right  and  left  bronchi,  one  of  which 
goes  to  each  lung.  The  trachea  consists  of  an  external 
fibrous  membrane,  between  the  layers  of  which  are  a  num- 
ber of  cartilaginous  rings,  and  an  internal  mucous  mem- 
brane. The  rings  are  the  most  striking  part  of  the  tra- 
chea. They  serve  to  keep  the  canal  open  at  all  times.  The 
inspiratory  effort  would  otherwise  collapse  the  walls  and 
prevent  the  entrance  of  air.  These  rings  are  sixteen  to 
twenty  in  number,  and  are  lacking  in  the  posterior  third  or 
fourth  of  the  circumference.  They  are,  therefore,  not  true 
rings.  The  interval  between  their  ends  is  filled  with  fibrous 
and  non-striped  muscular  tissue.  The  mucous  membrane  is 


136 


RESPIRATION 


lined  by  ciliated  epithelium,  and  has  mucous  glands  in  its 
substance  (Figs.  47,  48). 

The  Bronchi. — The  primitive  bronchi  are  of  the  same  es- 
sential structure  as  the  trachea.  The  right  is  the  larger, 
shorter,  and  more  nearly  horizontal.  This  probably  ac- 
counts for  the  more  frequent  lesions  in  the  right  lung.  Pen- 


BrorteMal  Musc/e. 


Bronchial '  flrtery. 


G/anc/ acini  &  cfucf. 

FIG.  49. 

B,   intra-pulmonary   bronchus   of   cat;   P. A.   and   P.V.,   pulmonary   artery   and 
vein;  bv,  bronchial  vein;   V ',  air  vesicles.      (Stirling.) 


etrating  the  lung  substance  they  divide  and  subdivide  until 
each,  by  its  ramifications,  communicates  with  every  air  vesi- 
cle in  that  lung.  When  the  primitive  bronchus  has  divided,  the 
incomplete  cartilaginous  rings  are  replaced  by  irregular 
plates  of  cartilage,  which  are  so  arranged  as  to  completely 
encircle  the  tube.  These  extend  as  far  as  the  division  of 
the  tubes  into  branches  %o  in.  in  diameter. 

Surrounding  the  tubes  in  the  lung  substance  is  a  circular 
layer  of  plain  muscular  fibers;  these  cease  only  at  the  air 


AIR  VESICLES  137 

vesicles.  Elastic  fibrous  tissue  is  also  present  everywhere  in 
the  bronchial  walls  and  is  continued  over  the  vesicles  them- 
selves. 

Bronchial  tubes  above  Vm  in.  in  diameter  have  in  their 
walls  cartilaginous  plates,  muscular  tissue,  fibrous  elastic 
and  inelastic  tissue  and  a  lining  membrane  of  ciliated  epi- 
thelium. 

Bronchial  tubes  l/5o  in.  in  diameter,  and  smaller,  have  in 
their  walls  the  same  elements  except  the  cartilage;  but  as  the 
tubes  subdivide,  their  walls  grow  continuously  thinner  and 
the  inelastic  tissue  becomes  less  and  less  in  amount  until  it 
finally  practically  disappears;  the  ciliated  epithelial  cells 
gradually  give  place  to  a  single  layer  of  squamous  cells  in 
the  smallest  tubes.  The  smallest  bronchial  tubes,  the  bron- 
chioles, are  from  ^20  to  M"o  in.  in  diameter.  O'f  course  ev- 
erywhere in  the  walls  there  are  vessels  and  nerves. 

The  Air  Vesicles. — Each  bronchiole  opens  into  a  collection 
of  air  vesicles,  or  cells,  called  a  pulmonary  lobule.  The  term 
lobulette  will  be  here  applied  to  it,  however,  reserving  the 
word  lobule  for  a  collection  of  lobulettes  about  *%  in.  in  di- 
ameter. The  bronchiole  entering  the  lobulette  becomes  the 
infundibulum  (Fig.  50),  a  slightly  dilated  canal  from  which 
are  given  off  from  eight  to  sixteen  oblong  vesicles,  the  true 
air  cells.  The  cells  are  a  little  deeper  than  they  are  wide  and 
end  in  blind  extremities.  The  diameter  of  the  lobulette  is 
about  %o-%2  in. ;  that  of  the  vesicle  about  ^oo-^o  in.  It  has 
been  estimated  that  there  are  some  725,000,000  of  these  ves- 
icles in  the  lungs  and  that  their  combined  area  is  something 
over  two  hundred  square  yards. 

The  walls  of  the  air  cells  are  very  thin,  being  composed  of 
a  single  layer  of  -flattened  epithelium  together  with  highly 
elastic  fibrous  tissue.  Ramifying  in  this  latter  is  a  most 
abundant  supply  of  capillaries,  which  are  larger  here  than 
anywhere  else  in  the  body.  The  physical  conditions  are  most 
favorable  for  the  exchange  of  gases  between  the  blood  and 
air,  each  capillary  being  exposed  to  vesicles  on  both  sides, 


138  RESPIRATION 

and  the  air  and  blood  being  separated  only  by  the  very  thin 
walls  of  the  capillary  and  vesicle.  The  elastic  tissue  is  very 
important  in  expelling  the  air  from  the  cells  when  the  in- 
spiratory  effort  has  ceased. 

For  the  nutrition  of  the  bronchi  and  lung  substance  ar- 


FIG.  50.-—  Terminal  branch  of  a  bronchial  tube,  with  its  infundib- 
ula  and  air-sacs,  from  the  margin  of  the  lung  of  a  'monkey,  injected 
with  quicksilver. 

a,  terminal  bronchial  twig;  b,  b,  air-sacs;  c,  c,  infundibula.  X  10.  (Kirkes 
after  E.  E.  Schulse.) 

terial  blood  is  furnished  by  the  bronchial  artery,  which  en- 
ters and  ramifies  with  the  bronchi.  The  entire  mass  of 
venous  blood  passes  directly  from  the  heart  through  the 
pulmonary  artery  to  the  lungs  to  be  arterialized,  and  it  is 
the  capillaries  of  this  artery  which  furnish  the  abundant  net- 
work between  the  air  cells. 

The  lungs  have  the  shape  of  irregular  cones,  their  bases 
resting  on  the  diaphragm  and  their  apices  extending  to 
points  a  little  above  the  clavicles.  They  are  completely  sep- 
arated from  each  other  by  the  mediastinum  and  their  exter- 
nal surfaces  are  covered  by  the  pleura,  a  serous  membrane 
similar  to  the  peritoneum  and  reflected  from  the  thoracic 
wall.  The  right  lung  is  divided  by  fissures  into  three  lobes 
and  the  left  into  two.  Superficially  the  lung  substance  is 
seen  to  be  subdivided  into  areas  about  Y$  in.  in  diameter 
called  the  lobules.  Each  lobule  is  composed  of  a  number  of 
lobulettes  as  above  mentioned. 


MECHANISM  OF  RESPIRATION  139 

MECHANISM  OF  RESPIRATION. 

Respiration  implies  the  more  or  less  regular  entrance  and 
exit  of  air  to  and  from  the  lungs.  The  entrance  is  inspira- 
tion; the  exit  expiration.  Now,  the  thorax  is  a  closed  cav- 
ity, notwithstanding  the  fact  that  the  lungs  have  an  opening 
(the  trachea)  by  which  they  communicate  with  the  external 
air;  and,  so  far  as  the  simple  ingress  and  egress  of  air  is 
concerned,  the  question  of  pulmonary  respiration  resolves 
itself  into  one  of  pure  mechanics.  The  lungs  may  be  looked 
upon  as  a  bag  (or  two  bags)  in  the  thoracic  cavity.  Inspired 
air  does  not  enter  the  thoracic  cavity,  but  this  bag  which  is 
in  it.  This  fact  is  of  the  greatest  importance. 

Furthermore,  the  lungs  are  everywhere  in  contact  with  the 
thoracic  wall  by  their  pleural  surfaces.  They  are  composed 
very  largely  of  highly  developed  elastic  tissue,  but  are  per- 
fectly passive  themselves.  That  is  to  say,  they  possess  no 
power  of  expansion  except  in  obedience  to  extraneous  in- 
fluences. As 'found  in  the  thorax  they  possess  a  contractile 
power,  but  only  because  certain  forces  have  put  their  elastic 
tissue  on  the  stretch,  and  the  contraction  is  a  simple  effort 
of  the  tissue  to  return  to  the  condition  which  characterized 
it  before  it  was  subjected  to  t]je  expanding  force. 

Before  birth  there  is  no  air  in  the  lungs,  and  this  is  the 
only  time  when  the  elastic  tissue  is  not  on  the  stretch.  The 
bronchioles  and  air  cells  are  collapsed,  but  the  thorax  is  con- 
tracted and  the  pulmonary  and  thoracic  walls  are  in  contact 
by  their  respective  pleural  surfaces.  When  the  child  is  born 
an  inspiration  fills  the  lungs  and  they  are  never  thereafter 
devoid  of  air.  They  collapse  to  a  certain  extent  and  leave 
the  thoracic  wall  when  the  chest  is  opened,  but  cannot  empty 
themselves  entirely  because  the  walls  of  the  bronchioles  col- 
lapse before  all  the  air  can  escape.  This  collapse  of  the 
lungs  when  the  chest  wall  is  opened  shows  that  the  lung 
structure  is  in  a  constant  state  of  tension,  which  tension  has 
always  a  tendency  to  empty  the  lungs,  but  cannot  do  so  be- 


I4O  RESPIRATION 

cause  the  thorax  can  contract  only  so  far,  and  when  its  con- 
traction has  reached  its  limit,  for  the  lung  to  contract  far- 
ther would  mean  a  separation  of  the  pulmonary  and  tho- 
racic walls  and  the  formation  of  a  vacuum  between  them. 
The  additional  reason  above  given,  namely  the  collapse  of 
the  bronchioles  before  all  the  air  can  escape,  is  inoperative 
under  normal  conditions  and  need  not  be  considered. 

Causes  of  Respiratory  Movements. — Seeing  that  the  lung 
structure  has  always  a  tendency  to  empty  itself  of  air,  it 
follows  that  inspiration  cannot  be  dependent  upon  the  lung 
itself.  Granting,  from  the  physical  conditions  present,  that 
the  lungs  and  thorax  must  expand  and  contract  together,  the 
expansion  of  the  lungs  in  inspiration  is  a  consequence  and 
not  a  cause  of  the  thoracic  expansion,  and  contraction  of  the 
lungs  in  expiration  is  a  cause  and  not  a  consequence  of  tho- 
racic contraction.  This  statement  as  to  expiration  applies 
only  to  ordinary  tranquil  respiration,  as  will  be  seen  later. 
Speaking  broadly  then,  inspiration  is  an  active  and  expira- 
tion a  passive  process.  That  is,  inspiration  occurs  as  a  re- 
sult of  the  activity  of  certain  muscles  which  operate  to  ex- 
pand the  thorax,  and  expiration,  as  a  consequence,  simply 
of  the  cessation  of  activity  on  the  part  of  those  muscles  and 
the  passive  contraction  of  the  lung  tissue. 

The  relation  of  the  thorax  and  lungs  and  the  action  of 
each  in  respiration  may  be  illustrated.  Suppose  a  bellows, 
which,  say  for  some  mechanical  reason,  cannot  completely 
collapse  and  which  is  itself  air-tight,  to  contain  a  thin  rubber 
bag.  communicating  by  a  tube  with  the  external  air ;  suppose 
the  bag  conforms  in  general  outline  to  the  shape  of  the  bel- 
lows, and  under  a  moderate  degree  of  distention  completely 
fills  the  cavity  of  the  bellows  when  the  latter  is  collapsed  as 
far  as  possible.  Now,  it  being  understood  that  the  bag  was 
somewhat  distended  to  cause  it  to  fill  the  bellows,  and  that 
all  air  .has  been  allowed  to  escape  by  a  temporary  opening 
from  between  the  walls  of  the  two  and  the  bellows  itself 
made  air-tight  afterwards,  it  follows  that  unless  the  bellows 


INSPIRATION  141 

can  contract  the  bag  will  remain  distended  and  will  not  leave 
the  bellows  wall,  although  it  will  have  a  constant  tendency 
to  do  so.  It  is  also  apparent  that,  since  the  bag  exerts  a  con- 
tinual compressing  effect  on  its  contents,  the  pressure  inside 
it  will  be  greater  than  that  outside  between  it  and  the  bellows 
wall.  Under  these  conditions  there  will  be  a  constant  ten- 
dency on  the  part  of  the  bellows  to  collapse,  and  some  active 
force  will  be  necessary  to  expand  it ;  when  it  is  made  to  ex- 
pand the  contained  bag  will  expand  with  it.  Suppose 
the  expansion  should  be  stopped  at  a  certain  point  and  the 
bellows  held  (to  prevent  contraction)  ;  it  is  obvious  that  now 
the  pressure  inside  the  bag  is  greater,  while  that  outside  be- 
tween its  walls  and  those  of  the  bellows  is  less,  than  when 
the  expansion  began;  that  is,  the  bag  has  become  distended 
more  and  is  exerting  a  greater  compressing  effect  upon  its 
contents.  If  now  the  bellows  be  simply  released,  both  the 
bag  and  the  bellows  will  contract  and  the  former  will  empty 
itself  so  far  as  the  latter  will  allow;  but  when  the  bellows 
has  reached  the  limit  of  its  contraction  the  bag  also  ceases 
to  contract,  although  it  remains  in  a  constant  state  of  ten- 
sion. If  at  any  time  air  be  admitted  to  the  bellows  proper 
the  bag  will  at  once  collapse. 

This  illustration  can  be  applied  to  the  mechanical  princi- 
ples obtaining  in  ordinary  respiration.  The  bellows  is  the 
air-tight  thorax  which  cannot  contract  beyond  a  certain 
point ;  the  rubber  bag  is  the  elastic  lungs  under  constant  ten- 
sion, communicating  by  the  trachea  with  the  external  air 
and  following,  or  being  followed  by,  the  movements  of  the 
thorax;  the  pressure  in  the  bag  and  between  it  and  the  bel- 
lows wall  represents  the  intrapulmonary  and  intrathoracic 
pressures  respectively. 

It  will  be  noticed  later  that  this  illustration  does  not  go 
quite  far  enough  to  explain  a  few  of  the  phenomena  of  ex- 
piration, but  it  could  very  easily  be  made  to  do  so. 

Inspiration. — Any  force  which  expands  the  thorax  aids 
in  inspiration;  and  any  muscles  which  increase  any  of  the 


142  RESPIRATION 

thoracic  diameters  expand  the  thorax.  The  diameters  in- 
creased are  chiefly  the  (i)  vertical,  and  (2)  ant ero- posterior. 

The  vertical  is  increased  by  descent  of  the  diaphragm, 
which  descent  is  caused  by  its  contraction,  since,  owing  to 
the  intra-thoracic  "pull"  exerted  upon  it,  it  is  normally 
vaulted  upward. 

The  antero-posterior  diameter  is  increased  chiefly  by  the 
elevation  of  the  ribs.  Since  these  bones,  attached  posteriorly 
to  the  spinal  column,  run  not  only  forward  but  also  down- 
ward to  join  the  sternum  by  the  costal  cartilages,  it  follows 
that  the  elevation  of  their  anterior  ends  will  increase  the  di- 
ameter in  question. 

Muscles  of  Inspiration. — Elevation  of  the  ribs  is  effected 
by  a  number  of  muscles.  The  three  scaleni  are  attached 
above  to  the  cervical  vertebrae  and  below  to  the  first  and  sec- 
ond ribs;  their  action  elevates  not  only  these  ribs  but  the 
whole  anterior  chest  wall. 

The  action  of  the  intercostales  externi  is  still  a  subject  of 
dispute  in  connection  with  the  physiology  of  respiration. 
These  muscles  are  attached  externally  to  the  adjacent  bor- 
ders of  the  ribs,  and  thus  occupy  the  intercostal  spaces. 
Their  fibers  are  directed  downward  and  forward,  and  the 
effect  of  contraction  of  any  single  intercostal  muscle  would 
be  to  approximate  the  two  ribs  to  which  it  is  attached ;  but 
if  it  can  be  assumed  that  the  first  rib  is  fixed,  then,  from 
the  direction  of  their  fibers,  the  external  intercostals  will 
render  the  ribs  more  nearly  horizontal  by  raising  their  an- 
terior movable  extremities.  It  seems  that  the  first  rib  is  pre- 
vented from  descending,  probably  by  the  simultaneous  con- 
traction of  the  scaleni.  The  intercostales  interni  have  a  di- 
rection almost  at  right  angles  to  that  of  the  externi ;  the  ster- 
nal portions  of  these  act  from  the  sternum  and  also  elevate 
the  anterior  extremities  of  the  ribs.  The  levatores  costarum 
are  attached  to  the  transverse  processes  of  the  dorsal  verte- 
brae and  to  the  upper  borders  of  the  ribs  posteriorly.  The 
transverse  processes  are  fixed  points  and  the  ribs  are  mov- 


EXPIRATION  143 

able  on  their  spinal  articulations.  Contraction  of  these  mus- 
cles is,  therefore,  very  efficient  in  elevating  the  anterior  ends 
of  the  ribs. 

The  action  of  the  diaphragm  is  the  most  notable  of  the 
muscular  phenomena  connected  with  respiration,  and  it  de- 
serves to  be  called  the  "muscle  of  respiration." 

These  are  the  muscles  which  are  chiefly  concerned  in  ordi- 
nary inspiration.  Their  combined  action  also  increases 
slightly  the  transverse  diameter  of  the  chest.  But  there  are 
certain  others,  known  as  auxiliary  muscles  of  inspiration, 
which  are  called  into  play  during  profound  or  forced  in- 
spiration. Their  action  is  evident  from  their  attachments — 
all  operating  chiefly  to  increase  the  antero-posterior  diame- 
ter. They  are  the  serratus  posticus  superior,  sterno-mastoi- 
deus,  levator  anguli  scalpula,  trapezius,  pectoralis  minor, 
pectoralis  major  (costal  portion),  serratus  magnus,  rhom- 
boidei  and  erectores  spines.  It  will  be  noticed  that  several  of 
these  which  usually  take  their  point  on  the  chest,  as,  for  ex- 
ample, the  sterno-mastoideus,  pectorales,  etc.,  must,  in  order 
to  aid  inspiration,  take  their  fixed  points  at  their  other  ex- 
tremities. 

Expiration. — When  the  force  which  expands  the  chest 
during  inspiration  ceases  to  operate,  expiration  follows. 
Not  only  does  the  elastic  (i)  lung  tissue  force  out  the  air, 
but  the  (2)  thoracic  walls,  by  their  costal  cartilages  and  their 
intercostal  tissues,  are  themselves  elastic,  and  this  elasticity, 
aided  by  the  (3)  "tone"  of  the  muscles  which  have  been  put 
upon  the  stretch  during  inspiration  and  which  are  now  seek- 
ing to  return  to  their  normal  condition,  tends  to  restore  the 
thorax  to  the  dimensions  it  had  previous  to  the  inspiratory 
act.  So  far  no  actual  muscular  contraction  has  been  brought 
into  play,  and  it  is  here  assumed  that  none  is  usually  con- 
cerned in  the  expiratory  act  of  ordinary  tranquil  respiration. 

Some  maintain  that  the  costal  portions  of  the  intercostales 
interni  particularly  are  expiratory  in  quiet  breathing;  they 
do  contract  and  the  ribs  approach  each  other  during  the 


144  RESPIRATION 

act,  but  it  is  probable  that  they  serve  only  to  maintain  the 
proper  degree  of  tension  of  the  intercostal  tissues. 

Although  the  elastic  reaction  of  the  lung  tissue  during  ex- 
piration operates  together  with  the  elasticity  of  the  thoracic 
wall  in  diminishing  the  antero-posterior  diameter  of  the 
chest,  it  is  chiefly  effective  in  diminishing  the  vertical  diam- 
eter by  raising  the  diaphragm.  It  exerts  a  certain  "suction" 
upon  that  muscle,  causing  it  to  arch  upward  in  following  the 
contracting  lungs.  It  is  seen,  therefore,  that  during  inspira- 
tion the  chest  wall  and  diaphragm  exert  "suction"  upon  the 
lungs,  causing  them  to  follow,  and  during  expiration  the 
lungs  exert  "suction"  upon  the  chest  wall  and  diaphragm, 
causing  them  to  follow. 

Forced  Expiration. — It  is  evident  that,  while  ordinary  ex- 
piration is  a  passive  process,  a  person  can  voluntarily  force 
out  of  his  lungs  more  air  than  is  ordinarily  expelled,  as  in 
singing,  blowing,  talking,  etc.  This  is  effected  by  certain 
muscles  whose  contraction  diminishes  the  thoracic  capacity, 
chiefly  by  depressing  the  ribs  and  elevating  the  diaphragm. 
Those  which  depress  the  ribs  are  the  intercostales  internl, 
infracostales  and  triangularis  sterni.  Those  which  elevate 
the  diaphragm  do  so  by  compressing  the  abdominal  contents 
and  forcing  them  up  against  that  muscle.  They  are  the  ob- 
liquus  externus,  obliquus  internus  transver sails  and  rectus 
abdominis.  These  depress  the  chest  wall  as  well. 

Rhythm  of  Respiration. — Under  ordinary  conditions  in- 
spiration and  expiration  follow  each  other  in  a  regular  rhyth- 
mical fashion.  Some  hold  that  an  interval  follows  inspira- 
tion before  expiration  begins,  but  this  is  probably  not  cor- 
rect. Indeed,  it  is  doubtful  if  there  be  an  interval  following 
expiration,  though  it  will  be  here  considered  that  there  is  a 
brief  one.  Expiration  is  a  little  longer  than  inspiration.  The 
inspiratory  act  is  of  uniform  intensity  throughout,  while  the 
expiratory  act  gradually  diminishes  in  intensity  as  it  ap- 
proaches completion — a  circumstance  to  be  expected  from 
the  physical  condition  causing  it. 


RATE  OF  RESPIRATION  145 

After  every  six  to  ten  respiratory  acts  a  more  profound 
(sighing)  inspiration  than  usual  is  taken,  the  effect  being  a 
more  thorough  changing  of  the  pulmonary  contents.  Cough- 
ing, sneezing,  hiccoughing,  laughing,  etc.,  all  interfere  with 
rhythmical  respiration. 

Modified  Respiration. — In  coughing  and  sneezing  a  pro- 
found inspiration  precedes  a  violent  convulsive  contraction 
of  the  expiratory  muscles.  Sighing  is  an  expression  on  the 
part  of  the  tissues  that  more  oxygen  is  needed  and  that, 
therefore,  the  contents  of  the  lungs  must  be  more  completely 
changed.  Yawning  is  a  phenomenon  similar  to  sighing,  but 
may  not  represent  deficient  oxygenation,  as  when  it  occurs 
from  contagion.  Except  in  the  contraction  of  different  fa- 
cial muscles,  sobbing  and  laughing  are  identical  from  a  re- 
spiratory standpoint;  in  both  there  is  a  succession  of  quick 
contractions  of  the  diaphragm.  Hiccough  is  an  involuntary 
contraction  of  the  diaphragm  accompanied  by  closure  of  the 
glottis.  It  takes  place  during  inspiration.  In  hawking  the 
glottis  is  open  and  a  continuous  expiratory  current  is  sent 
through  the  narrowed  passage  between  the  base  of  the 
tongue  and  the  soft  palate.  Snoring  occurs  with  the  mouth 
open ;  the  current  of  air  throws  the  uvula  into  vibration  and 
produces  the  characteristic  sounds. 

Sounds  of  Respiration. — When  the  ear  is  applied  to  the 
chest  there  is  heard  during  inspiration  a  breezy  expansive 
sound  of  slightly  increasing  intensity  throughout,  and  ceas- 
ing abruptly  at  the  end  of  the  act.  Immediately  begins  the 
expiratory  sound,  very  short,  lower  in  pitch  than  the  inspira- 
tory,  and  gradually  decreasing  in  intensity  until  it  is  lost 
before  expiration  is  more  than  one-fourth  finished.  When 
listening  over  a  large  bronchus  this  sound  is  prolonged  and 
has  a  higher  pitch  than  usual.  Respiratory  sounds  are  more 
pronounced  in  the  female  than  in  the  male  chest,  owing  to 
the  predominance  of  costal  breathing  in  the  former  sex. 

Rate  of  Respiration. — The  respiratory  rate  sustains  a  fair- 
ly constant  relation  to  the  cardiac  rate,  the  ratio  being  about 
10 


146  RESPIRATION 

one  to  four.  This  makes  the  average  number  of  respirations 
about  eighteen  per  minute  for  adults.  In  a  general  way  this 
rate  is  subject  to  variations  from  the  same  causes  as  that  of 
the  pulse.  Any  appreciable  .fall  in  the  amount  of  oxygen  in 
the  inspired  air  will  increase  the  number  of  respirations  for 
obvious  reasons.  The  frequency  and  depth  usually  bear  an 
inverse  ratio  to  each  other. 

Types  of  Respiration. — (i)  Costal  respiration  is  that  car- 
ried on  by  the  chest  walls;  (2)  diaphragmatic,  that  effected 
by  the  diaphragm.  In  the  former  type  movements  of  the 
thorax  are  concerned ;  in  the  latter,  movements  of  the  abdo- 
men. According  as  the  movements  in  costal  respiration  are 
more  pronounced  in  the  upper  or  lower  segment  of  the  chest, 
that  type  is  subdivided  into  (a)  superior  costal,  and  (b)  in- 
ferior costal. 

In  young  children  the  diaphragmatic,  or  abdominal,  type 
prevails ;  in  adult  males  a  combination  of  the  inferior  costal 
and  abdominal;  in  adult  females  the  superior  costal.  The 
last  circumstance  is  probably  due  in  part  to  the  mode  of 
dress  in  civilized  countries,  and  in  part  to  the  provision 
against  encroachment  of  the  uterus  upon  the  abdominal 
cavity  during  pregnancy. 

Intrapulmonary  and  Intrathoracic  Pressure. — It  is  evi- 
dent that  during  inspiration  the  pressure  inside  the  lungs 
(intrapulmonary)  is  less  than  the  ordinary  atmospheric 
pressure ;  this,  in  fact,  is  the  immediate  cause  of  the  entrance 
of  air.  It  is  also  evident  that  during  expiration  the  intrapul- 
monary pressure,  owing  to  the  compressing  effect  of  the 
lung  tissue  and  the  thoracic  walls,  is  greater  than  the  outside 
atmospheric  pressure ;  this  is  the  immediate  cause  of  the  exit 
of  air.  In  both  acts  the  air  rushes  in  or  out,  as  the  case  may 
be,  in  an  effort  to  maintain  the  same  pressure  inside  the 
lungs  as  exists  in  the  surrounding  atmosphere.  It  is  con- 
venient to  call  the  pressure  which  is  less  than  atmospheric 
negative,  and  that  which  is  greater  positive  pressure. 

The  intrapulmonary  pressure  is  negative  during  inspira- 


PULMONARY   CAPACITY  147 

tion  and  positive  during  expiration.  Now,  owing  to  condi- 
tions already  referred  to,  as  the  chest  and  lungs  expand  dur- 
ing inspiration,  the  pressure  between  the  adjacent  walls  of 
the  two  (intrathoracic)  becomes  less  and  less  and  reaches 
a  minimum  at  the  end  of  that  act.  Furthermore,  owing  to 
the  continuous  "pull"  of  the  elastic  lungs  upon  the  chest 
walls  the  intrathoracic  pressure  remains  negative  even  at  the 
end  of  expiration.  But  it  can  be  made  to  become  positive 
under  forced  action  of  the  expiratory  muscles,  as  in  cough- 
ing, blowing,  etc.  The  constantly  increasing  negative  con- 
dition of  intrathoracic  pressure  is  evidenced  by  a  drawing 
in  of  the  intercostal  tissues  during  inspiration;  when  the 
pressure  assumes  a  positive  character,  as  in  the  expiratory 
acts  of  the  pulmonary  emphysema,  these  tissues  bulge  out- 
ward. 

Pulmonary  Capacity. — It  is  evident  that  the  most  forcible 
expiration  cannot  completely  empty  the  lungs  of  air.  The 
air  remaining  after  such  an  effort  is  the  residual  air.  It 
amounts  to  about  100  cubic  inches.  But  in  ordinary  respira- 
tion at  the  end  of  the  expiratory  act  there  is  more  than  100 
cubic  inches  of  air  in  the  lungs,  because  in  such  cases  all  the 
air  possible  is  not  forced  out.  In  fact  about  200  cubic  inches 
usually  remain ;  this  consists  of  the  residual  plus  another  100 
cubic  inches,  which  is  called  the  reserve  or  supplemental 
air.  It  can  be  forced  out,  but  is  not  in  tranquil  respiration. 
The  amount  of  air  which  is  taken  into  the  lungs  by  an  ordi- 
nary respiratory  act  amounts  to  about  20  cubic  inches,  and  is 
termed  tidal  air.  It  is  the  only  volume  used  in  quiet  breath- 
ing. At  the  end  of  the  inspiratory  act  in  tranquil  respira- 
tion it  is  obvious  that  the  expansion  may  continue  still  far- 
ther, and  a  certain  amount  of  air,  over  and  above  the  tidal 
air,  be  taken  into  the  lungs.  The  maximum  amount  which 
can  be  so  inspired  (beyond  the  tidal)  is  about  no  cubic 
inches,  and  is  the  complemental  air. 

It  is  seen,  then,  that  the  entire  lung  capacity  is  equal  to 
about  330  cubic  inches.  But  the  residual  air  cannot  under 


148  RESPIRATION 

any  circumstances  be  called  into  use,  and  consequently  the 
vital  capacity  is  equal  to  the  total  capacity  minus  the  residual 
air  (100  cubic  inches),  or  230  cubic  inches.  It  is  the  volume 
which  can  be  expelled  by  the  most  forcible  expiration  after 
the  most  forcible  inspiration. 

The  capacity  of  the  trachea  and  larger  bronchi  is  known 
as  the  bronchial  capacity,  and  amounts  to  about  8  cubic 
inches. 

The  quantity  of  air  in  the  small  bronchioles  and  air  vesi- 
cles is  increased  by  inspiration  and  decreased  by  expiration; 
it  is  called  alveolar  capacity,  and  at  the  end  of  ordinary  ex- 
piration amounts  to  about  150  cubic  inches.  Quiet  inspira- 
tion increases  it  to  about  180  cubic  inches. 

All  these  estimates,  of  course,  represent  only  an  aver- 
age. The  vital  capacity  is  increased  by  stature,  by  any  oc- 
cupation which  calls  for  active  physical  work  and  by  various 
other  conditions. 

Composition  of  Air. — Ordinary  atmospheric  air  contains, 
in  round  numbers,  about  21  parts  of  oxygen  to  79  parts  of 
nitrogen.  These  two  gases  make  up  the  main  bulk  of  the  at- 
mosphere. In  addition,  the  atmosphere  always  contains  a 
little  carbon  dioxide  (about  .04  per  cent.),  ammonia,  mois- 
ture, organic  material,  dust,  nitric  acid,  etc.  All  except  the 
oxygen  and  nitrogen  are  of  minor  importance  in  respiration 
when  they  are  not  present  in  amounts  beyond  the  usual.  It 
will  be  seen  that  the  striking  difference  between  inspired  and 
expired  air  is  in  the  proportions  of  oxygen  and  carbon  diox- 
ide. 

Diffusion  in  the  Lungs. — The  expired  air  contains  much 
more  CO  and  much  less  O  than  the  inspired  air.  The  inter- 
change of  gases  between  the  alveolar  air  and  the  blood  is 
responsible  for  the  difference. 

The  question  is  what  forces  cause  the  O  of  the  air  to  enter 
the  alveoli  and  the  CCte  to  leave  it.  As  might  be  supposed, 
the  air  escaping  during  the  first  part  of  expiration  differs 
very  little  in  composition  from  the  inspired  air,  for  it  has 


DIFFUSION  IN  THE  LUNGS  149 

been  occupying  the  upper  air  passages  where  no  interchange 
occurs.  The  bronchial  capacity  is  only  about  one-third  large 
enough  to  accommodate  the  tidal  air,  and  consequently  the 
greater  part  of  it  must  come  from  lower  down  in  the  lung 
structure,  and  the  CO2  in  the  expired  air  continuously  in- 
creases until  the  end  of  the  act.  At  each  inspiration  at  least 
two-thirds  of  the  tidal  air  must  pass  into  the  small  bronchi, 
or  lower.  Thus  it  is  that  inspiration  and  expiration  them- 
selves, taking  into  and  bringing  out  of  the  vesicles  (or  at 
least  the  bronchioles)  air  fresh  with  O  and  air  vitiated 
with  CO2,  aid  very  materially  in  keeping  constant  the  com- 
position of  the  alveolar  air. 

In  the  second  place,  the  cardiac  movements  have  a  similar 
effect,  each  systole  decreasing  the  size  of  the  heart  and  in- 
ducing a  fresh  atmospheric  current  toward  the  deep  alveoli, 
and  each  diastole  forcing  a  like  current  of  vitiated  air  toward 
the  trachea.  This  force  is  not  inconsequential. 

In  the  third  place,  the  diffusibility  of  gases  under  known 
physical  laws,  without  the  aid  of  any  such  movements  as 
have  been  described,  is  an  occurrence  in  connection  with  the 
phenomenon  in  question.  Every  gas,  under  ordinary  atmos- 
pheric conditions,  exerts  a  certain  pressure.  In  every  me- 
chanical mixture  of  gases  (such  as  the  atmosphere)  each  in- 
dividual gas  exerts  a  part  of  the  total  pressure — a  part  pro- 
portional to  its  percentage  in  that  mixture.  This  has  been 
called  the  "partial  pressure"  of  that  gas.  Since  O  is 
present  in  ordinary  atmosphere  to  the  extent  of  21  parts  per 
hundred,  the  partial  pressure  of  oxygen  in  the  atmosphere 
is  21/ioo  of  the  total  pressure. 

Now,  in  the  air  of  the  alveoli  O  is  present  to  a  less  extent 
than  21  parts  per  hundred,  and  consequently  its  partial  pres- 
sure in  that  situation  is  less  than  in  the  trachea  and  bronchi. 
The  result  is  that  O  continually  makes  its  way  from  the 
point  of  higher  pressure  (trachea  and  bronchi)  toward  the 
point  of  lower  pressure  (alveoli).  The  tendency  is  thus  to 
establish  a  uniform  partial  pressure  throughout  the  whole 


I5O  RESPIRATION 

respiratory  tract;  but  this  is  never  done  during  life  because 
the  partial  pressure  above  is  being  continually  increased  by 
the  introduction  of  new  O,  and  below  is  being  continually 
diminished  by  the  removal  of  that  gas  from  the  alveoli  by 
the  blood. 

In  case  of  CO2  opposite  conditions  prevail.  This  gas  is 
being  continually  introduced  into  the  alveolar  air  from  the 
blood,  and  consequently  it  is  present  there  in  much  larger 
quantities  than  in  the  trachea  and  bronchi,  which  contain 
newly  inspired  air.  The  partial  pressure,  therefore,  of  CO2 
in  the  alveoli  is  much  higfrer  than  in  the  upper  respiratory 
passages,  and  a  continual  current  of  it  diffuses  upward  to 
equalize  the  pressure;  this  is  never  accomplished,  however, 
for  reasons  of  similar  nature  to  those  keeping  up  the  con- 
stantly unequal  pressure  of  O. 

These  three  factors — respiratory  and  cardiac  movements 
and  the  natural  diffusion  of  gases — are,  therefore,  in  con- 
tinual operation  to  get  O  to  and  CO2  away  from  the  alveoli. 
Under  their  influence  the  composition  of  the  alveolar  air  re- 
mains fairly  uniform. 

Alterations  of  Air  in  the  Lungs. — These  are  chiefly :  (a) 
Loss  of  oxygen,  (b)  gain  of  carbon  dioxide,  (c)  elevation 
of  temperature,  (d)  gain  of  water,  (e)  gain  of  ammonia,  (/) 
gain  of  organic  matter,  (g)  gain  of  nitrogen,  (h)  loss  of 
(actual)  volume.  The  capital  changes  are  loss  of  O  and 
gain  of  CO2. 

(a)  Loss  of  Oxygen. — The  air  in  passing  through  the 
lungs  loses  of  O  nearly  5  per  cent,  of  its  total  volume.  That 
is,  whereas  on  entering  it  contains  21  parts,  on  leaving  it 
contains  only  about  16  parts  per  hundred  of  this  gas.  Nearly 
25  per  cent,  of  the  total  volume  of  O  inspired,  therefore,  is 
lost  in  the  lungs. 

When  the  respirations  are  18  to  the  minute,  and  20  cu.  in. 
of  air  are  inspired  at  each  breath,  the  amount  inspired  in  an 
hour  will  be  21,600  cu.  in.  Since  a  little  more  than  one-fifth 
of  this  air  is  O,  and  since  only  'one-fourth  of  the  inspired  O 


ALTERATIONS  OF  AIR   IN   THE  LUNGS 


is  consumed,  the  total  amount  necessary  for  an  hour  will  be 
about  1,100  cu.  in.  This  allows,  however,  for  no  muscular, 
digestive  or  other  activity,  and  the  amount  actually  necessary 
is  larger  than  this. 

The  circumstances  which  call  for  an  increase  in  O  almost 
invariably  cause  an  increase  in  the  output  of  C(X 

(b)  Gain  of  Carbon  Dioxide. — The  amount  of  CCte  in  in- 
spired air  is  about  .04  part  per  hundred  (fioo  per  cent.)  ;  the 
amount  in  expired  air  is  something  more  than  4  parts  per 
hundred.  In  round  numbers  then,  the  air  in  passing  through 
the  lungs  gains  of  CO2  4  per  cent,  of  its  entire  volume. 
This  is  in  periods  of  rest  from  exercise,  digestion,  etc.  The 
total  amount  discharged  in  one  hour  is,  on  an  average,  about 
1,000  cu.  in.  This  estimate  should  probably  be  raised  to 
1,200  cu.  in.  for  ordinary  activity,  and  varies  according  to 
many  conditions,  some  of  which  are  rapidity  and  depth  of 
respiration,  age,  sex,  digestion,  diet,  sleep,  exercise,  mois- 
ture, temperature,  season,  integrity  of  the  nerve  supply,  etc. 

The  subjoined  table  from  Kirkes'  Physiology  compares 
the  composition  of  inspired  and  expired  air. 


Inspired  Air. 

Expired  Air. 

Oxygen                   .  .  . 

2096  vols    per  cent. 

1  6  03  vols   per  cent 

Nitrogen  

79       vols.  per  cent. 

79       vols   per  cent 

Carbonic  acid 

o  04  vols   per  cent. 

4  4    vols   per  cent 

Watery  vapor  

variable 

saturated 

Temperature 

variable 

that  of  body  (36°  C  ) 

Conditions  Influencing  Output  of  CO 2. — When  the  ra- 
pidity of  respiration  is  increasing,  the  depth  remaining  con- 
stant, the  percentage  of  CCte  in  the  expired  air  is  reduced 
because  more  air  is  respired,  but  the  total  quantity  in  any 
given  time  is  increased.  The  same  result  follows  an  in- 
creased depth  and  a  constant  rate.  With  a  diminished  ra- 


152  RESPIRATION 

pidity  and  increased  depth  more  CO2  is  exhaled  than  under 
opposite  conditions. 

The  amount  of  CO  exhaled  is  small  in  very  young  in- 
fants. But  soon  the  output  begins  to  increase,  and  in  males 
continues  to  do  so  up  to  about  thirty  years ;  there  is  then  a 
slight  decrease  up  to  sixty,  and  afterward  a  considerable  de- 
crease to  death. 

In  the  female  the  output  is  less  than  in  the  male.  In  the 
former  sex  the  increase  is  said  to  cease  at  puberty  and  to 
remain  constant  until  the  menopause,  after  which  time  it  in- 
creases to  sixty  and  diminishes  subsequently. 

During  digestion  the  quantity  is  considerably  increased. 
This  is  probably  due  to  the  muscular  activity  of  the  alimen- 
tary tract,  to  glandular  metabolism  and  to  changes  taking 
place  in  the  food  products. 

As  to  diet,  it  may  be  said  in  general  that  the  exhaled  CQz 
is  increased  in  quantity  by  the  taking  of  nitrogenized  foods, 
tea  and  coffee. 

The  influence  of  sleep  is  to  diminish  the  output. 

Muscular  exercise  is  very  efficient  in  increasing  the 
amount  of  CCte  exhaled ;  in  fact,  this  explains  partly  the  va- 
riations in  connection  with  sex,  digestion,  sleep,  etc. 

A  high  degree  of  moisture  increases  the  exhalation,  as 
does  a  rise  in  body  temperature.  A  rise  in  external  tempera- 
ture, however,  has  an  opposite  effect. 

The  output  is  increased  in  spring  and  decreased  in  autumn. 

When  the  efferent  nerve  supplying  a  part  is  severed  the 
production  of  COs  in  that  part  is  at  once  diminished. 

The  consumption  of  O  and  the  exhalation  of  CCte  bear  a 
fairly  constant  relation  to  each  other — any  condition  in- 
creasing one  increasing  the  other,  and  vice  versa.  The 
facts,  therefore,  which  have  been  mentioned  as  governing 
the  exhalation  of  CO2  may  be  applied  to  the  consumption 
of  O. 

(c)  Gain  in  Temperature. — When  the  body  temperature 
is  normal  and  the  external  atmospheric  temperature  about 


OXYGE-N  CONSUMED  AND  CARBON  DIOXIDE  EXHALED      153 

70°  F.,  it  is  found  that  air  inspired  through  the  nose  and  ex- 
pired through- the  mouth  has  its  temperature  raised  from  70° 
to  about  95° ;  the  rise  is  less  when  the  inspiration  takes  place 
through  the  mouth.  The  last  air  of  expiration  is  warmer 
than  the  first.  This  gain  of  heat  while  the  air  is  in  the  lungs 
needs  no  explanation  when  it  is  remembered  that  the  aver- 
age temperature  of  the  tissues  with  which  it  is  in  contact  is 
98.5°  F.,  or  higher. 

(d)  Gain  of  Water. — This  water  is  in  the  form  of  vapor. 
It  is  natural  that  the  air  should  absorb  water  from  the  moist 
surfaces  with  which  it  is  in  contact.    The  capillary  network 
with  which  it  is  in  close  relation  supplies  moisture  to  the  mu- 
cous membrane  not  only  of  the  alveoli  but  of  the  entire 
respiratory  tract.     One  or  two  pounds  of  water  are  elimi- 
nated thus  daily. 

(e)  Gain  of  Ammonia. — Ammonia  is  exhaled   in  small 
quantity  by  the  lungs.     It  is  insignificant  except  in  cases  of 
suppressed  kidney  action. 

(/)  Gain  of  Organic  Matter. — The  quantity  of  organic 
matter  exhaled  by  the  lungs  is  inconsequential  (unless  venti- 
lation be  bad),  but  such  exhalation  does  occur  to  a  small  ex- 
tent. It  gives  the  odor  to  the  breath. 

(g)  Gain  of  Nitrogen. — The  exhalation  of  this  gas  by  the 
lungs  is  of  no  respiratory  importance.  The  amount  is  said 
to  be  Moo4£o  the  amount  of  oxygen  consumed.  An  occa- 
sional loss  of  nitrogen  has  been  observed. 

(h)  Decrease  of  (Actual)  Volume. — When  the  external 
temperature  is  below  about  90°  F.  the  volume  of  expired  air 
is  a  little  greater  than  that  of  the  inspired  air,  because  of  the 
increase  of  temperature  it  undergoes  in  passing  through  the 
lungs.  But  the  actual  volume  of  the  expired  air,  when  re- 
duced to  the  same  temperature  as  the  inspired,  is  found  to  be 
always  a  little  less  than  that  of  the  latter.  It  is  estimated 
that  from  #o-%o  of  the  total  volume  of  the  inspired  air  is 
thus  lost  in  respiration. 

Besides  the  substances  mentioned  as  being  exhaled  from 


154  RESPIRATION 

the  lungs,  it  is  well  known  that  odorous  emanations  proceed 
from  them  after  garlic,  onions,  turpentine,  alcohol,  certain 
drugs,  etc.,  have  been  taken  into  the  stomach. 

Relation  Between  Oxygen  Consumed  and  Carbon  Dioxide 
Exhaled. — A  given  volume  of  O  will  combine  with  carbon 
to  form  the  same  volume  of  CO ;  or  the  amount  of  O  in  a 
given  volume  of  CO  is  equivalent  to  that  volume  when  set 
free  from  the  carbon.  A  cubic  foot  of  O  will  unite  with 
carbon  to  form  a  cubic  foot  of  CCte;  or  a  cubic  foot  of 
CO2  will  yield,  on  dissociation,  a  cubic  foot  of  O. 

This  being  the  case,  if  all  the  O  consumed  in  the  lungs 
were  exhaled  therefrom  in  the  form  of  CO,  the  amount  of 
CO  exhaled  would  just  equal  the  amount  of  O  consumed. 
But  the  amount  of  consumed  O  is  about  5  per  cent,  of  the 
inspired  air,  while  the  amount  of  exhaled  CO  is  only  about 
4  per  cent,  of  the  expired  air.  It  follows,  therefore,  that  I 
per  cent,  of  the  volume  of  inspired  air  is  not  represented  by 
the  CO2  exhaled  from  the  lungs  and  skin.  The  relation  be- 
tween the  consumed  O  and  the  exhaled  COv  is  usually  ex- 
pressed as  the  "respiratory  quotient" — the  division  of  the 
latter  by  the  former  giving  the  quotient.  This  quotient  is 
made  to  vary  by  many  circumstances,  though  for  any  con- 
siderable period  its  average  is  about  the  same. 

While  it  has  been  stated  that  the  O  absorbed  and  the  CO2 
produced  vary  together  usually,  they  are  in  a  certain  meas- 
ure independent  of  each  other.  For  CO  does  not  result 
from  the  immediate  union  of  O  with  carbon  of  the  carbo- 
hydrates and  fats,  but  may  be  stored  in  the  shape  of  com- 
plex compounds,  which  may  later  split  up  with  the  formation 
of  CO2,  either  by  oxidation  or  by  intramolecular  cleavage. 
Furthermore,  more  O  is  necessary  to  oxidize  (that  is,  to 
form  carbon  dioxide)  some  molecules  than  others.  A  fat 
requires  considerably  more  O  to  produce  CO2  than  does  a 
carbohydrate;  so  that  the  kind  of  food  in  store  would 
also  affect  the  respiratory  quotient. 

With  respect  to  the  O  which,  in  the  long  run,  is  not  repre- 


CONDITION  OF  CO2  IN  THE  BLOOD  155 

sented  in  the  CO2  exhaled  from  the  lungs  and  skin,  it  is 
certain  that  when  various  of  the  food  stuffs  are  broken  down 
at  least  a  part  of  it  is  appropriated  by  hydrogen  to  form 
water. 

Source  of  Exhaled  Carbon  Dioxide. — The  increase  of  CO2 
in  expired  air  over  the  small  amount  contained  in  inspired 
air  is  derived  from  the  venous  blood  circulating  through  the 
lungs.  It  exists  in  that  blood  under  a  constant  tension,  as  is 
demonstrated  by  its  escape  when  the  blood  is  placed  in  a 
vacuum.  The  total  amount  escapes  when  the  blood  intact  is 
placed  in  vacua :  when  the  corpuscles  alone  are  so  treated 
they  yield  up  all  their  OCte,  though  it  is  small  in  amount; 
but  the  plasma  alone  in  vacuo  yields  a  less  amount  than 
when  it  contains  corpuscles.  If  now  corpuscles  be  added  to 
the  plasma  the  total  amount  of  OO2  is  forthcoming.  The 
corpuscles  must,  therefore,  act  as  an  acid  causing  the  liber- 
ation of  this  gas  from  the  plasma.  It  is  probably  the  hemo- 
globin, or  oxyhemoglobin,  which  has  this  effect,  though  in 
the  laboratory  the  phosphates  and  certain  proteids  of  the 
corpuscles  produce  a  like  reaction  when  brought  in  contact 
with  the  carbonates  and  bicarbonates  of  soda. 

Condition  of  CCte  in  the  Blood. — About  5  per  cent,  of  the 
total  amount  of  OCte  in  venous  blood  is  in  simple  solution 
in  the  plasma;  about  75-85  per  cent,  is  in  loose  chemical 
combination  in  both  corpuscles  and  plasma;  the  remaining 
10-20  per  cent,  is  in  comparatively  stable  combination  in  the 
plasma.  Of  the  75-85  per  cent.,  by  far  the  largest  part  is  in 
the  plasma,  probably  in  a  condition  of  loose  association  with 
sodium  to  form  carbonates  and  bicarbonates ;  the  small  part 
in  the  corpuscles  may  exist  in  a  similar  state,  but  it  is  now 
thought  to  exist  in  combination  with  the  proteid  portion  of 
hemoglobin.  The  total  75-85  per  cent,  in  corpuscles  and 
plasma  is  so  loosely  combined  that  the  mere  diminution  in 
pressure  in  the  lungs  is  probably  sufficient  to  liberate  it. 
The  10-20  per  cent,  in  firm  chemical  combination  is  that  part 
which  cannot  be  extracted  from  plasma  alone  in  vacuo,  but 


156  RESPIRATION 

which  is  dissociated  on  the  addition  of  an  acid,  or  corpuscles, 
or  hemoglobin,  etc.  It  may  be  that  as  the  blood  passes 
through  the  lungs  there  is  set  free,  in  the  formation  of  oxy- 
hemoglobin,  an  acid  which  immediately  unites  with  the  bases 
holding  the  CO  in  combination —  the  liberation  of  the  latter 
being  the  consequence. 

The  O  being  thus  in  the  air  vesicles,  and  the  CO  thus 
free,  or  set  free,  in  the  blood,  with  the  very  thin  animal  mem- 
brane consisting  of  the  vesicular  and  capillary  walls  between 
them,  it  remains  to  be  seen  what  forces  are  concerned  in  the 
interchange  of  these  gases.  It  has  been  noted  that  only  one- 
fourth  of  the  O  entering  the  lungs  in  the  air  is  taken  up  by 
the  blood ;  so  it  is  to  be  remembered  that  not  all  the  CCte 
entering  the  lungs  in  the  venous  blood  is  taken  up  by  the  air. 

Interchange  of  Oxygen  and  Carbon  Dioxide  in  the  Lungs. 
—The  condition  of  "partial  pressure"  of  gases  in  mixture 
has  been  mentioned.  Each  gas  exerts  a  pressure  in  propor- 
tion to  its  percentage  in  the  mixture,  and  this  is  called  its 
"partial  pressure."  Now,  the  extraction  of  O  and  COs  from 
the  blood  by  placing  it  in  a  vacuum  shows  that  both  these 
gases  exist  in  the  blood  under  a  certain  degree  of  tension. 

The  tension  of  a  gas  in  solution  being  only  the  pressure 
necessary  to  keep  it  in  solution,  it  follows  that  if  the  pres- 
sure be  diminished  the  gas  will  partly  escape.  If  an  atmos- 
phere containing,  say,  O  at  a  certain  partial  pressure  be 
Drought  in  contact  with  a  fluid  containing  O  at  a  certain 
tension,  unless  the  partial  pressure  of  the  O  in  -the  air  be 
equal  to  its  tension  in  the  fluid  there  will  be  an  escape  of  the 
gas  from  the  point  of  higher  to  the  point  of  lower  pressure 
or  tension.  If  the  partial  pressure  of  the  gas  be  less  in  the 
atmosphere  than  its  tension  in  the  fluid,  the  current  will  be 
from  the  latter  to  the  former  and  vice  versa.  This  will  be 
the  case  whether  the  media  are  in  actual  contact  or  separ- 
ated by  an  animal  membrane. 

This  is  the  condition  which  obtains  in  the  pulmonary  alve- 
oli. The  partial  pressure  of  O  in  the  alveolar  air  is  much 


CONDITION  OF  OXYGEN    IN   THE  BLOOD  157 

greater  than  the  tension  of  O  in  the  blood ;  consequently  the 
current  is  from  the  air  to  the  blood.  The  tension  of  CO2  in 
the  venous  blood  is  much  greater  than  the  partial  pressure 
of  the  CQz  in  the  alveolar  air;  consequently  the  current  is 
from  the  blood  to  the  air. 

But,  here,  as  in  the  last  analysis  of  almost  all  physiolog- 
ical phenomena,  it  is  found  that,  while  these  purely  physical 
laws  are  certainly  concerned  in  the  pulmonary  interchange 
of  gases,  they  are  insufficient  to  explain  the  occurrence  in 
full.  For  the  blood  will  take  from  the  alveolar  air  more  than 
enough  O  to  establish  an  equilibrium  of  tension  and  partial 
pressure;  the  tension  of  O  in  arterial  blood  is  higher 
than  its  partial  pressure  in  alveolar  air.  So  it  is 
found  that  the  alveolar  air  will  remove  more  than  enough 
CO2  to  establish  a  similar  equilibrium  of  this  gas.  It  is 
known  that  the  avidity  (chemical)  of  corpuscles  for  O  to 
form  oxyhemoglobin  causes  the  blood  to  appropriate  more 
O  than  it  would  otherwise  do,  but  even  then  we  are  driven  to 
the  usual  ultimatum  of  ascribing  some  peculiar  office  to  the 
living  epithelium  of  the  intervening  membrane. 

Condition  of  Oxygen  in  the  Blood. — Almost  all  the  oxy- 
gen is  conveyed  in  the  blood  by  the  red  corpuscles,  where 
it  exists  in  rather  unstable  composition  with  hemoglobin 
(probably  with  its  pigment  portion)  under  the  name  of  oxy- 
hemoglobin. Only  a  comparatively  small  part  is  held  in  so- 
lution by  the  plasma.  Dissociation  of  oxyhemoglobin  oc- 
curs when  the  pressure  is  sufficiently  reduced. 

Alterations  in  Blood  in  Passing  Through  the  Lungs. — 
The  sum  total  of  the  changes  taking  place  in  the  blood  as  it 
passes  through  the  lungs  is  represented  by  the  term  arteriali- 
zation.  In  general,  it  may  be  said  that  the  blood  undergoes 
changes  exactly  opposite  to  those  of  the  air  in  circulating 
through  the  pulmonary  structure,  and  reference  to  the  list 
of  substances  gained  and  lost  by  the  air  will  suggest  the 
main  alterations  in  the  blood. 

Of  course  the  most  striking  phenomena  are  the  loss  of 


158  RESPIRATION 

CO2  and  the  gain  of  O.  In  100  volumes  of  arterial  or  venous 
blood  there  are  found  to  be,  on  an  average,  60  volumes  of  O 
and  CO2.  This  total  remains  approximately  constant,  though 
the  relative  amount  of  each  gas  varies  according  as  the 
blood  is  venous  or  arterial,  and  in  venous  blood  under  the 
influence  of  several  conditions  to  be  mentioned.  In  arterial 
blood  the  O  will  represent  about  20,  and  the  COn  about  40, 
of  the  total  60  volumes  per  hundred  of  gas.  In  ordinary 
venous  blood  the  O  will  represent  about  7  volumes  less  (13) 
and  the  CO  about  7  volumes  more  (47)  of  the  total  60.  In 
both  venous  and  arterial  blood  there  is  an  insignificant 
amount  of  nitrogen,  which  is  usually  present  to  the  extent 
of  1.5  volumes  per  hundred. 

The  proportion  of  gases  is  about  the  same  in  arterial 
blood  taken  from  any  part  of  the  system.  In  blood  coming 
from  actively  secreting  glands  the  ratio  of  O  to  CO2  is 
nearly  the  same  as  in  arterial  blood ;  in  fact,  such  blood  may 
have  a  red  (arterial)  instead  of  a  blue  (venous)  color.  This 
is  because  during  activity  blood  is  sent  to  the  gland  in  in- 
creased amount  to  furnish  materials  for  secretion,  while  the 
demand  for  oxygen  is  not  relatively  increased  in  that  gland. 

Besides  the  changes  which  are  apparent  on  referring  to 
the  alterations  in  the  air  passing  through  the  lungs,  there 
are  certain  other  general  characteristics  which  distinguish 
arterial  from  venous  blood.  The  most  noticeable  is  color. 
Venous  blood  is  changed  in  the  lesser  circulation  from  a  dark 
blue,  or  black,  to  a  bright  red.  This  is  due  to  the  formation 
of  oxyhemoglobin.  The  change  of  color  does  not  occur 
when  the  appropriation  of  O  is  interfered  with,  as  when  the 
air  is  excluded  from  the  lungs,  or  when  carbon  monoxide  is 
inhaled.  .  Again,  there  is  every  reason  to  believe  that  venous 
blood  coming  from  different  organs  differs  in  composition 
according  to  the  special  materials  which  have  been  extracted 
from  it  by  those  organs;  the  portal  blood  during  digestion 
must  certainly  be  different  in  composition  from  the  general 
venous  blood,  and  so  it  may  be  conceived  that  the  blood  com- 


INTERNAL  RESPIRATION  159 

ing  from  no  two  different  sets  of  capillaries  is  identical. 
When  all  this  meets  in  the  right  side  of  the  heart  and  is  sent 
thence  into  the  lungs  it  has  a  nearly  uniform  composition, 
and  needs  only  to  receive  O  before  it  can  supply  the  wants 
of  any  particular  tissue  in  the  body.  Arterial  blood  is  also 
more  coagulable  than  venous. 

Internal  Respiration. — It  has  been  said  that  the  object  of 
external  respiration  and  the  transportation  of  O  and  CCte  is 
to  make  internal  respiration  possible.  Oxygen,  leaving  the 
alveoli  in  a  manner  already  described,  enters  the  blood  and 
at  once  combines  with  hemoglobin  of  the  red  corpuscles 
to  form  oxyhemoglobin.  A  small  portion  of  the  O  is  used 
up  by  the  corpuscles  in  transit,  with  the  production  of  CO2 
and  other  metabolic  materials — the  corpuscles  requiring  O 
in  their  metabolism  just  as  do  other  cells.  But  by  far  the 
largest  portion  is  carried  to  the  capillaries,  where  it  is  taken 
up  by  the  cells.  At  the  same  time  the  cells  give  up  to  the 
blood  CO2 — a  result  of  their  metabolic  activity.  The  blood, 
having  thus  given  up  its  O,  is  changed  in  color,  and  carries 
the  CO2  back  to  the  lungs  to  be  exhaled. 

To  furnish  O  and  to  remove  CO  is  the  only  object  of 
respiration.  Living  tissue  exposed  to  an  atmosphere  con- 
taining O  will  consume  O  and  exhale  CO  even  if  no  blood 
be  circulating  through  it.  The  exact  manner  in  which  a  cell 
uses  O  is  not  apparent.  It  is  evidently  an  oxidation  process, 
which  produces  CC)2,  and  O  is  directly  necessary  to  this  pro- 
cess. But  the  amount  of  CCte  produced  in  any  given  time 
may  not  correspond  to  the  amount  of  O  consumed  in  that 
time ;  it  may  be  greater  or  less.  "It  is  probable  that  during 
rest  O  is  utilized  to  some  extent  in  oxidations  which  are  not 
at  once  carried  to  their  final  stage  and  in  which  relatively 
little  CO2  is  formed ;  hence  during  activity  comparatively 
little  O  is  required  to  cause  a  final  disintegration  of  the  now 
partially  broken  down  substances,  and  thus  to  give  rise  to  a 
relatively  large  formation  of  COz"  (Reichert). 

The  absorption  of  O  is  to  be  looked  upon  as  a  part  of  the 


l6o  RESPIRATION 

nutritive  process  just  as  the  absorption  of  proteid,  e.  g., 
and  COs  as  one  of  the  products  of  destructive  metabolism 
just  as  urea.  There  is  small  probability  that  the  O  unites 
directly  with  the  carbon  of  any  of  the  food  stuffs — although 
this  is  the  final  result. 

Interchange  of  Oxygen  and  Carbon  Dioxide  in  the  Tis- 
sues.— Here  application  of  the  principles  governing  the  in- 
terchange of  these  gases  in  the  lungs  applies.  It  is  found 
that  the  tissues  act  as  very  strong  reducing  agents  upon  oxy- 
hemoglobin,  setting  free  the  O.  Now  the  tension  of  O  in  the 
arterial  capillaries  is  much  higher  than  in  the  tissues ;  in  fact, 
it  is  practically  nothing  in  the  latter  situation,  for  the  O  en- 
ters so  quickly  into  combination  that  there  is  very  little  to  be 
found  here  at  any  time.  Consequently  physical  laws  en- 
courage the  passage  of  this  gas  out  of  the  capillaries  into 
the  tissue. 

On  the  other  hand,  the  tension  of  COa  in  the  tissues  is 
much  higher  than  in  the  blood,  and  the  same  physical  laws 
"encourage  a  current  of  CO2  toward  the  blood.  Neverthe- 
less, these  laws  do  not  explain  all  the  phenomena  of  inter- 
change ;  the  activity  of  the  cells  is  an  important  agent,  though 
their  influence  may  be  of  a  chemical  nature  only. 

Cutaneous  Respiration. — Cutaneous  respiration  in  man  is 
insignificant  and  not  essential  to  life.  The  skin  absorbs  a 
little  O  and  exhales  a  little  more  CCte.  It  is  estimated  by 
Scharling  that  the  skin  performs  about  %o  of  the  respiratory 
function.  Death  following  the  covering  of  the  body  surface 
with  an  impermeable  coating  is  not  due  to  interference  with 
cutaneous  respiration. 

Ventilation. — Persons  breathing  in  a  confined  space  grad- 
ually consume  the  O  and  increase  the  OOa  of  the  atmosphere. 
When  the  amount  of  O  has  been  decreased  to  fifteen  parts 
per  hundred  it  is  insufficient  for  the  respiratory  demands. 
When  the  OCte  is  increased  to  .07  part  per  hundred  the  air 
becomes  disagreeable  and  close;  this  is  not,  however,  from 
the  accumulation  of  CO  so  much  as  from  organic  emana- 


RESPIRATION  OF  VARIOUS  GASES  l6l 

tions  and  disagreeable  odors  from  the  body,  clothing,  etc. 
It  is  only  that  the  amount  of  CCte  serves  as  an  indication  of 
the  extent  of  accumulation  of  these  materials  that  the 
amount  of  .07  per  cent,  is  fixed  as  the  limit  beyond  which  it 
ought  not  to  be  present.  This  percentage  of  COs  in  air  free 
from  emanations,  etc.,  is  not  deleterious. 

Since  1,200  cu.  in.  of  O  are  consumed  per  hour,  about  15 
cu.  ft.  will  be  necessary  for  a  day ;  and  since  the  1,200  cu.  in. 
consumed  represent  only  about  one-fourth  of  the  O  inspired, 
60  cu.  ft.  will  be  necessary  ior  inspiration  during  twenty- 
four  hours.  This  amount  represents  some  300  cu.  ft.  of  at- 
mospheric air — which  an  ordinary  person  must  have  in  that 
time. 

But  this  estimate  allows  nothing  for  increased  respiratory 
activity,  which  inevitably  occurs  from  some  of  the  numerous 
conditions  influencing  it.  It  is  found  that  in  prisons  and 
other  institutions  of  confinement  it  is  not  safe  to  allow  each 
person  less  than  1,000  cu.  ft.  of  atmospheric  air.  In  crowded 
houses,  where  this  space  per  individual  cannot  be  obtained, 
it  is  necessary,  in  order  to  avoid  unpleasant  results,  to  change 
the  air  continuously,  or  at  frequent  intervals.  Natural  and 
artificial  means  are  employed  to  accomplish  this  end. 

Respiration  of  Various  Gases. — The  inhalation  of  pure 
oxygen  is  not  deleterious  unless  it  be  under  higher  tension 
than  in  atmospheric  air,  when  it  becomes  a  local  irritant. 
The  blood  will  not,  however,  appropriate  more  than  the  us- 
ual amount.  Nitrous  oxide  will  sustain  respiration  for  a 
time,  but  soon  produces  unconsciousness  and  asphyxia,  prob- 
ably because  it  unites  so  firmly  with  the  hemoglobin  of  the 
corpuscles.  Hydrogen  may  be  inhaled  with  impunity  if  it 
contain  also  oxygen  in  the  atmospheric  proportion.  Carbon 
monoxide  is  poisonous  because  it  unites  with  hemoglobin  to 
the  exclusion  of  oxygen  and  will  not  dissociate  itself.  Sul- 
phuretted, phosphoretted  and  arseniuretted  hydrogen  are  de- 
structive of  hemoglobin  and  consequently  poisonous.  Pure 
carbon  dioxide  cannot  be  inhaled  for  any  length  of  time, 
ii 


1 62  RESPIRATION 

Abnormal  Respiration. — The  term  eupnea  is  used  to  de- 
scribe normal,  tranquil  breathing.  Apnea  is  suspended  res- 
piration. Hyperpnea  is  exaggerated  respiration.  Dyspnea 
is  labored  breathing.  Asphyxia  is  essentially  a  want  of  O 
characterized  by  convulsive  respirations,  and  later  by  irregu- 
lar shallow  breathing.  The  last  two  named  deserve  some  at- 
tention. 

Dyspnea  may  be  due  to  either  a  deficiency  of  O  or  an  ex- 


FIG.  51. — The  heart  in  the  first  stage  of  asphyxia. 

The  left  cavities  are  seen  to  be  distended;  the  left  ventricle  partly  overlaps 
the  right;  La.,  left  auricle;  l.v.,  left  ventricle;  a,  aorta;  p.a.,  pulmonary  artery; 
p.v.,  pulmonary  vein;  r.a.,  right  auricle;  r.v.,  right  ventricle;  v.c.d..  descending 
vena  cava;  v.c.a.,  ascending  vena  cava.  (Kirkes  after  Sir  George  Johnson.) 

cess  of  CO2  in  the  blood.  When  an  animal  is  made  to 
breathe  in  a  small,  confined  space  the  amount  of  O  soon  be- 
comes insufficient  even  though  the  amount  of  CO2  in  the 
blood  be  not  increased.  Again,  if  an  animal  be  caused  to 
breathe  air  containing  the  usual  amount  of  O  and  a  large 
amount  of  CO2,  it  will  suffer  from  dyspnea  also.  In  either 
case  the  manifestations  are  practically  the  same — slow,  deep 
and  labored  respiration.  In  cardiac  disease,  hemorrhage, 
pulmonary  diseases,  etc.,  dyspnea  is  from  lack  of  O  in 
the  tissues,  because  of  enfeebled  action  of  the  heart,  deficient 


ASPHYXIA  163 

quantity  of  blood,  insufficient  exposure  of  the  blood  in  the 
lungs,  etc. 

Asphyxia  may  be  looked  upon  as  exaggerated  dyspnea. 
The  labored  breathing  of  dyspnea  becomes  convulsive,  and 
finally  collapse  ensues.  Respiration  becomes  shallow,  con- 
sciousness is  lost,  the  pupils  are  dilated,  opisthotonus  devel- 
ops, the  reflexes  disappear,  and  at  last  the  heart  stops  beat- 
ing. The  skin  and  mucous  membranes  become  blue  from 


FIG.  52. — 'The  heart  in  the  final  stage  of  asphyxia. 

The  letters  have  the  same  meaning  as  in  Fig.  51;  in  addition,  p.c.  represents 
the  pulmonary  capillaries.  The  right  auricle  and  ventricle,  and  the  pulmonary 
artery,  are  fully  distended,  while  the  left  cavities  of  the  heart  and  the  aorta 
are  nearly  empty.  (Kirkes  after  Sir  George  Johnson.) 

non-oxygenation  of  the  blood.  Asphyxia  from  submersion 
is  harder  to  overcome  than  from  simple  deprivation  of  air 
outside  the  water.  Resuscitation  is  extremely  doubtful  when 
a  person  has  been  submerged  as  long  as  five  minutes. 

While  the  phenomena  of  dyspnea  and  asphyxia  are  refer- 
able to  the  lungs,  it  is  not  the  need  of  air  in  these  organs,  but 
of  O  in  the  tissues,  which  gives  rise  to  the  symptoms.  The 
non-oxygenated  blood  in  asphyxia  will  not  circulate  through 
the  capillaries  except  with  the  greatest  difficulty,  and  the 
result  is  that  it  accumulates  in  the  arterial  system,  dams 


i64 


RESPIRATION 


back  upon  and  distends  the  heart,  so  that  this  organ  is  finally 
paralyzed  and  ceases  to  beat.  This  is  the  cause  of  death 
from  asphyxia. 

Effect  of  Respiration  on  Blood-Pressure. — The  lowest 
blood-pressure  is  just  after  the  beginning  of  inspiration, 
from  which  time  it  increases  during  inspiration  to  reach  its 
maximum  just  after  the  beginning  of  expiration;  it  gradu- 


FIG.  53. — Carotid  blood-pressure  tracing  of  a  dog. 

Vagi  not  divided;  I,  inspiration;  E,  expiration.     (Stirling.') 

ally  decreases  from  this  time  to  the  minimum  just  after  the 
beginning  of  inspiration.  The  general  effect,  then,  of  in- 
spiration is  to  increase  blood-pressure  and  of  expiration  to 
decrease  it.  This  remark  applies  to  general  arterial  tension. 

Taking  inspiration,  the  increase  in  arterial  tension  is,  in  its 
last  analysis,  due  to  the  larger  amount  of  blood  sent  into 
the  arterial  system  at  each  ventricular  systole.  The  explana- 
tion is  somewhat  complex,  but  if  the  mechanics  of  respira- 
tion be  understood  it  may  be  made  satisfactory. 

It  was  seen  that  the  lungs  are  contained  in  an  air-tight 
cavity,  the  chest,  and  that  they  expand  with  the  chest  be- 
cause of  negative  pressure  ("suction")  exerted  upon  them. 
The  heart  is  also  a  hollow  organ  situated  in  this  cavity;  it 
has  connected  with  it,  and  lying  also  in  the  thoracic  cavity, 
large  vessels  communicating  with  smaller  extrathoracic  ves- 
sels. The  heart  and  these  great  thoracic  vessels  are  elastic 
and  distensible.  Consequently  the  expansion  of  the  thorax 


RESPIRATION    AND    BLOOD-PRESSURE  165 

also  expands  them  slightly  and  tends  to  draw  blood  from 
the  extrathoracic  into  the  intrathoracic  vessels  and 
heart;  in  fact  inspiration  is  one  of  the  main  forces 
causing  a  flow  of  venous  blood  toward  the 
heart.  Now  all  this,  so  far  as  it  goes,  tends  to  keep  the 
blood  out  of  the  extrathoracic  vessels,  and  so  to  contradict 
the  statement  that  inspiration  increases  arterial  tension. 

But,  remembering  that  we  are  dealing  with  arterial  ten- 
sion and  that  our  effort  is  to  prove  that  more  blood  is  sent 
into  the  aorta  during  inspiration  than  during  expiration,  it 
is  of  value  to  note  that  since  the  walls  of  the  aorta  are  more 
resistant  than  those  of  the  venae  cavae  there  is  less  expansion 
of  the  former  than  of  the  latter  during  inspiration,  and  con- 
sequently less  tendency  for  the  arterial  blood  to  regurgitate 
into  the  thoracic  aorta  than  for  the  venous  blood  to  enter 
the  thoracic  venae  cavse.  The  same  expanding  force  dilates 
the  pulmonary  capillaries,  pulmonary  artery  and  pulmonary 
veins — the  artery  least  of  these.  Taking  it  for  granted  that 
more  blood  is  being  received  by  the  right  side  of  the  heart 
from  the  incoming  venae  cavae,  the  somewhat  dilated  pul- 
monary artery  receives  more  from  the  right  ventricle ;  the 
pulmonary  capillaries  are  more  dilated  than  the  artery  and 
this  fact  greatly  encourages  (by  a  suggestive  "suction")  the 
increased  flow  from  the  pulmonary  artery ;  they,  therefore, 
receive  more  blood  than  usual.  The  pulmonary  veins,  being 
likewise  dilated,  exert  "suction"  upon  the  capillaries,  and 
thus  receive  and  pass  on  to  the  heart  a  larger  supply  of 
blood  than  usual.  The  heart,  receiving  more  blood,  must 
send  more  into  the  aorta,  thereby  increasing  arterial  tension 
in  the  extrathoracic  vessels,  unless,  by  expansion  of  the 
chest,  the  thoracic  aorta  be  so  dilated  as  to  accommodate 
the  increased  amount — which  is  not  true. 

Then,  finally,  the  validity  of  this  argument  will  hinge  on 
the  relative  dilatation  of  the  thoracic  aorta  and  of  the  thor- 
acic venae  cavae.  If  the  veins  be  less  dilated  by  inspiration 
than  the  artery,  then  they  will  receive  an  increase  of  blood 


1 66  RESPIRATION 

which  will  not  completely  occupy  the  increase  of  space  in  the 
dilated  thoracic  aorta,  and  there  will  be  a  backward  "suc- 
tion" made  upon  the  contents  of  the  arterial  tree  with  a 
consequent  decrease  in  pressure;  but  a  condition  just  oppo- 
site to  this  seems  to  obtain. 

During  expiration  contrary  conditions  in  general  are  op- 
erative with  contrary  results.  The  intrapulmonary  vessels 
and  heart  are  compressed,  but  the  veins  and  capillaries  more 
than  the  aorta,  with  the  result  that  less  blood  reaches  the 
heart  than  during  inspiration,  and  the  thoracic  aorta  being, 
relatively  to  the  thoracic  venae  cavae,  more  dilated  now  than 
during  inspiration  can  easily  accommodate  the  decreased 
amount  of  blood  which  it  receives.  Of  course  expiration  in- 
creases venous  pressure  in  the  veins  which  enter  the  thorax 
back  as  far  as  the  valves. 

The  reason  the  pressure  does  not  rise  with  the  beginning 
of  inspiration  is  because  a  short  time  is  consumed  in  filling 
the  flaccid  intrapulmonary  veins,  and  the  first  increase  of 
blood  is  delayed  for  that  purpose  instead  of  passing  on  to 
the  left  side  of  the  heart.  Similarly,  the  pressure  continues 
to  rise  for  a  short  time  after  expiration  has  begun  because 
the  large  veins  are  being  emptied  by  pressure  during  this 
time  and  their  contents  are  reaching  the  heart  and  being 
forced  into  the  aorta. 

Movements  of  the  diaphragm  and  abdominal  muscles  dur- 
ing respiration  also  lend  themselves  to  create  like  changes  in 
arterial  pressure,  but  the  main  factors  are  intrathoracic. 

The  fact  that  the  cardiac  rate  is  increased  during  inspira- 
tion and  decreased  during  expiration  may  also  have  to  do 
with  the  variations  in  pressure. 

All  the  foregoing  remarks  relative  to  arterial  tension  are 
meant  to  apply  to  tranquil  respiration.  During  forced  in- 
spiration, or  forced  expiration,  the  results  may  be  modified, 
or  even-  reversed,  by  circumstances  not  necessary  to  mention. 

Nervous  Mechanism  of  Respiration. — Although  the  mus- 
cles of  respiration  are  of  the  striated  variety,  it  is  by  no  ef- 


NERVOUS  MECHANISM  OF  RESPIRATION  167 

fort  of  the  will  that  the  movements  are  kept  up.  They  belong 
to  the  class  known  as  automatic ;  that  is,  they  are,  up  to  cer- 
tain limits,  under  the  control  of  the  will,  but  recur  in  a  reg- 
ular, coordinate  and  orderly  manner  without  the  active  inter- 
vention of  volition.  Respiration  represents  the  activity  of  a 
self-governing  apparatus.  These  movements  constitute  a 
finely  coordinated  set  of  contractions — contractions  which 
are  regulated  by  means  of  afferent  and  efferent  nerves  under 
the  supervision  of  the  respiratory  center. 

The  respiratory  center  is  in  the  lower  part  of  the  medulla 
oblongata.  Destruction  of  the  encephalon  above,  or  the  cord 
below,  the  center  does  not  arrest  respiration.  It  is  bi- 
lateral— a  center  for  each  side — and  these  are  more  or  less 
independent  of  each  other,  but  are  so  intimately  connected 
by  commissural  fibers  that  any  impression  made  upon  one 
usually  produces  a  like  effect  upon  the  other.  Each  half  pre- 
sides over  the  lungs  and  respiratory  muscles  of  its  own  side, 
but  acts  synchronously  with  its  fellow  of  the  opposite  side. 
Furthermore,  each  of  these  lateral  centers  may  be  regarded 
as  consisting  of  two  parts,  one  for  inspiration  and  one  for 
expiration.  Stimulation  of  the  inspiratory  center  not  only 
strengthens  the  inspiratory  act,  but  also  accelerates  respira- 
tion. Stimulation  of  the  expiratory  center  strengthens  ex- 
piration and  also  retards  the  respiratory  rate.  The  acceler- 
ator portion  of  the  center  seems  more  sensitive  than  the  in- 
hibitory, and  the  result  of  stimulation  of  the  whole  center  ist 
therefore  quickened  respiration. 

Subsidiary  respiratory  centers  are  said  to  exist  in  the  tuber 
cinereum,  optic  thalamus,  corpora  quadrigemina,  pons  Va- 
rolii  and  spinal  cord ;  but  the  existence  of  at  least  some  of 
these  is  doubtful. 

Rhythm  of  Respiration. — 'What  agency  excites  the  center 
to  keep  up  the  respiratory  movements  with  such  regularity 
is  a  matter  of  interest.  The  chief  circumstances  which  seem 
to  affect  the  rate  and  rhythm  are  (i)  the  will,  (2)  emotions, 
(3)  composition  of  the  blood,  and  (4)  afferent  impressions. 


1 68  RESPIRATION 

i,  2.  The  effect  of  the  will  and  emotions  are  too  apparent 
to  call  for  comment.  I  and  2  are  properly  included  in  4. 

3.  A  deficiency  of  O  or  an  excess  of  CO2  in  the  blood  will 
increase  the  rate.    Increase  in  temperature  of  the  blood,  as 
in  fever,  will  produce  a  similar  effect. 

4.  The  most  important  of  these  agencies  is  found  in  affer- 
ent impressions  conveyed  to  the  center.    The  fibers  carrying 
these  impressions  are  chiefly  in  the  pneumo gastric,  glosso- 
pharyngeal,  trigeminal  and  cutaneous  nerves.    Of  these  the 
pneumogastric  is  by  far  the  most  important. 

Section  of  a  single  pneumogastric  is  followed  by  variable 
respiratory  disturbances  which  usually  disappear  in  less  than 
an  hour.  Section  of  both  nerves  is  followed,  after  a  short 
interval  of  increased  respiratory  activity,  by  slow  and  pow- 
erful inspirations,  by  forced  expiration  and  an  appreciable 
interval  before  the  next  inspiration.  Irritation  of  the  cen- 
tral end  of  the  cut  nerve  by  a  very  weak  current  seems  to 
stimulate  the  inhibitory  part  of  the  center,  for  the  rate  is 
slowed,  the  expirations  are  strenuous  and  the  inspirations 
weak.  When  the  current  is  increased  to  a  moderate  strength 
opposite  results  are  obtained,  the  accelerator  portion  of  the 
center  being  stimulated.  These  facts  show  that  the  pneu- 
mogastrics  possess  both  inspiratory  and  expiratory  fibers, 
and  that  the  former  are  stimulated  more  by  a  moderate  cur- 
rent and  the  latter  more  by  a  very  weak  one.  The  rhythm 
of  respiration,  therefore,  includes  the  regular  sequence  of 
inspiratory  and  expiratory  movements  upon  each  other. 

Now  what  is  it  that,  under  normal  conditions,  irritates  the 
terminals  of  the  pneumogastrics  and  causes  them  to  convey 
inspiratory  and  expiratory  impressions?  It  has  been  held 
that  a  change  in  the  composition  of  the  alveolar  air — an  ac- 
cumulation of  carbon  dioxide — irritates  the  nerve  terminals 
and  explains  the  conveyance  of  the  inspiratory  impressions, 
while  the  stretching  of  the  lung  tissue  originates  the  expira- 
tory impressions.  Others  ascribe  both  inspiratory  and  ex- 
piratory impressions  to  lung  movements — movements  of  in- 


NERVOUS  MECHANISM   OF  RESPIRATION  169 

spiration  exciting  expiratory  fibers,  and  movements  of  ex- 
piration exciting  inspiratory  -fibers.  These  observers  cite 
the  fact  that  artificial  inflation  and  aspiration  excite  expira- 
tion and  inspiration  respectively. 

Stimulation  of  the  superior  laryngeal,  as  when  foreign 
bodies  accidentally  enter  the  larynx,  excites  violent  expira- 
tion. 

The  glosso-pharyngeal  contains  afferent  fibers  especially 
important  in  arresting  respiration — at  any  stage  whatever — 
during  the  act  of  deglutition. 

Stimulation  of  the  sensory  fibers  of  the  trigeminal  in  the 
nose,  as  by  irritating  vapors,  may  arrest  respiration. 

Irritation  of  the  cutaneous  nerves  in  general,  as  by  cold  or 
hot  water,  slapping,  etc.,  stimulates  respiratory  movement. 

There  are,  of  course,  running  from  the  cortex  to  the  res- 
piratory center  intracranial  fibers  whereby  the  organ  of  the 
will  makes  its  presence  felt  in  respiration. 

But  when  all  the  afferent  nerve  connections  are  severed, 
respiration  continues  with  modified  rhythm  and  rate,  at  least 
for  a  time.  It  is  thought  that,  under  these  conditions,  it  is 
the  circulation  through  the  center  of  blood  deficient  in  oxy- 
gen which  causes  the  cells  to  discharge ;  that  is,  after  every 
inspiration  and  subsequent  expiration  there  is  not  another 
inspiration  until  the  blood  has  become  sufficiently  deoxygen- 
ated,  or  charged  with  carbon  dioxide,  to  irritate  the  respira- 
tory center. 

We  may  conclude  that  "the  rhythmical  discharges  from 
the  center  are  due  primarily  to  an  inherent  quality  of  peri- 
odic activity  of  the  nerve  cells  constituting  the  respiratory 
center,  and  maintained  by  the  blood,  and  that  the  rhythm, 
rate,  and  other  characters  of  these  discharges  may  be  af- 
fected by  the  will  and  the  emotions,  by  the  composition,  sup- 
ply and  temperature  of  the  blood,  and  by  various  afferent  im- 
pulses. The  chief  factors  are  the  quantities  of  O  and 
CO2  in  the  blood,  and  the  impulses  conveyed  from  the  lungs 
by  the  fibers  of  the  pneumogastric  nerves."  (Am.  Text- 
.Book.) 


170  RESPIRATION 

The  efferent  nerves  of  respiration  control  the  muscular 
movements  of  that  act.  They  are  chiefly  the  facial,  hypo- 
glossal  and  spinal  accessory  controlling  the  respiratory 
movements  about  the  face  and  throat,  the  pneumo gastric 
going  to  the  larynx  and  the  phrenic  to  the  diaphragm. 

To  the  lungs  proper  fibers  are  distributed  by  the  vagus, 
the  dorsal  sp.inal  and  the  sympathetic  nerves.  Besides  the 
expiratory  and  inspiratory  fibers  already  noticed,  the  vagus 
supplies  the  lungs  with  broncho-motor,  general  sensory,  tro- 
phic and  secretory  (mucous)  fibers.  The  sympathetic  fur- 
nishes trophic  and  vaso-motor  fibers,  which  latter  come  from 
the  cord  by  the  roots  of  the  dorsal  nerves  mentioned  to  join 
the  sympathetic  ganglia. 


CHAPTER  IX. 

NUTRITION,  DIETETICS  AND  ANIMAL  HEAT. 
NUTRITION. 

ALL  the  processes  of  the  body  as  digestion,  absorption,  se- 
cretion, circulation,  respiration,  etc. — have  a  single  object, 
viz.,  the  nutrition  of  the  cells  of  the  body. 

The  ultimate  source  of  all  nutriment  is,  of  course,  food 
and  oxygen.  The  oxygen  has  been  followed  from  the  lungs 
to  the  tissues  as  oxyhemoglobin  of  the  blood.  The  various 
foods  have  been  seen  to  disappear  from  the  digestive  tract 
and  to  be  conveyed  to  the  tissues  by  the  great  nutritive  fluid, 
some  in  recognizable  and  some  in  unrecognizable  form.  If, 
now,  we  shall  be  able  to  discover  in  what  way  these  different 
materials  thus  furnished  the  cells  are  utilized  and  appropri- 
ated by  them,  and  in  what  condition  they  subsequently  es- 
cape from  the  system,  the  study  of  nutrition  will  have  been 
rendered  much  clearer.  The  intake  is  through  the  lungs  and 
alimentary  canal ;  the  output  is  mainly  by  the  lungs,  skin,  kid- 
neys, and  intestines.  To  show  for  the  changes  which  take 
place  while  the  food  is  in  the  body  there  is  the  growth  of  the 
body,  the  maintenance  of  tissue  integrity,  secretion,  heat, 
motion  and  nervous  energy. 

It  may  be  said  at  once,  however,  that  the  exact  method  of 
appropriation  of  nutritive  material  by  the  tissues  is  a  sub- 
ject of  speculation,  since  it  involves  the  question  of  life 
itself ;  and  we  shall  have  to  be  content  with  recounting  some 
of  the  conditions  influencing  and  some  of  the  phenomena 
attendant  upon  the  process. 

171 


172  NUTRITION,   DIETETICS   AND   ANIMAL    HEAT 

Metabolism. — By  metabolism  is  meant  those  processes  in 
the  body  whereby  food  products  are  appropriated,  their 
stored-up  energy  utilized,  and  the  waste  discarded. 

Metabolism  is  divided  into,  (i)  anabolism,  and  (2)  kata- 
bolism.  Anabolism  is  the  process  of  building  up  tissue  by 
cell  appropriation  of  food  stuffs.  Katabolism  is  the  process 
of  destroying  tissue  in  order  to  set  free  energy  that  the  or- 
gans of  the  body  may  perform  their  various  functions. 

When  the  anabolic  processes  are  equal  to  the  katabolic 
there  is  no  excessive  storage  of  material,  but  an  individual 
remains  of  uniform  size,  weight,  and  strength.  If  the  ana- 
bolic are  in  excess  of  the  katabolic  processes,  the  excessive 
products  are  stored  up  in  cells  and  an  individual  increases  in 
size,  weight  and  strength.  If  the  katabolic  processes  are  in 
excess  of  the  anabolic  there  is  a  call  on  the  tissues  for  the 
matter  already  stored  there  and  there  is  a  decrease  in 
strength,  weight  and  size. 

Death. — As  long  as  a  cell  appropriates  enough  to  supply 
the  deficit  caused  by  the  destruction  of  material  in  the  ex- 
penditure of  energy,  the  cell  will  live;  but  when  the  intake 
cannot  make  up  for  the  output  lost  the  cell  ceases  to  func- 
tionate and  this  is  called  death. 

Problems  Involved  in  the  Nutritive  Process. — Since  the 
actual  changes  occurring  and  the  method  of  their  produc- 
tion cannot  be  understood,  the  question  of  nutrition  resolves 
itself  into  a  consideration  of  the  final  fate  of  the  various 
aliments,  of  their  relative  value  in  nutrition,  of  conditions  in- 
fluencing the  process,  and  of  the  explanation  of  certain 
facts  connected  with  the  destruction  of  the  food  stuffs,  par- 
ticularly the  production  of  heat. 

The  change  which  the  foods  finally  undergo  in  the  body 
is  one  of  oxidation.  It  is  therefore  chemical  changes  which 
give  rise  to  physical  activity.  Oxidation  is  accompanied  by 
the  production  of  heat.  The  same  sum  total  of  heat  is  de- 
veloped when  a  piece  of  iron  rusts  completely  away  in  five 
years  as  when  it  is  consumed  in  an  atmosphere  of  oxygen  in 


FOODS  IN   NUTRITION  1/3 

five  minutes.  In  both  cases  it  is  oxidized.  In  the  cell  oxi- 
dation is  continually  going  on  with  the  production  of  heat 
and  of  certain  excrementitious  (oxidation)  products  de- 
pending on  the  kind  of  food  stuffs. 

Fate  of  Different  Foods  in  the  Organism. — In  the  first 
place,  the  foods  may  be  divided,  into  (I)  those  which  pass 
through  the  organism  unchanged,  and  (II)  those  which  lose 
their  identity  and  are  discharged  as  bodies  different  from 
those  which  entered.  The  first  class  includes  the  foods  fur- 
nishing no  energy;  the  second  those  furnishing  energy. 

Only  a  few  foods  undergo  in  the  body  reactions  which 
alter  their  identity.  They  may  be  regarded  as  already  di- 
gested and,  in  fact,  when  dissolved,  ready  for  discharge  from 
the  body.  They  are,  however,  useful  and  necessary  constitu- 
ents of  the  body,  and  if  they  do  not  take  a  considerable  active 
part  in  nutrition,  their  favorable  influence  on  that  process 
makes  them  essential  to  health.  The  foods  producing  no 
energy  may  be  dismissed  with  a  repetition  of  the  statement 
that  they  are  largely  introduced  in  connection  with  the  pro- 
teid  foods  from  which  they  cannot  be  separated  without  de- 
struction of  the  proteid  molecule.  Indeed,  all  the  proteid 
food  introduced,  whether  animal  or  vegetable,  contains  inert 
constituents  as  a  part  of  the  molecule,  and  these  seem  as 
necessary  to  nutrition  as  do  the  energy  furnishing  constitu- 
ents. The  foods  furnishing  energy  and  those  furnishing  no 
energy  enter,  are  deposited,  and  seem  to  be  discharged  both 
together.  The  few  reactions  which  the  inert  foods  undergo 
in  the  body  do  not  materially  affect  the  supply  of  energy. 

(II)  The  proteids,  carbohydrates  and  hydrocarbons  are 
all  consumed  in  the  organism,  none  (unless  they  have  acci- 
dentally escaped  digestion)  being  discharged  as  they  entered. 

i.  The  nitrogenous  foods  are  changed  into  peptones  in  the 
alimentary  canal,  undergo  some  unknown  change  in  their  ab- 
sorption therefrom,  appear  in  the  blood  as  the  proteid  con- 
stituents of  that  fluid,  and  are  offered  to  the  tissues  through 
the  medium  of  the  lymph.  The  complex  proteid  molecule  is 


174  NUTRITION,   DIETETICS   AND   ANIMAL    HEAT 

broken  down  into  simpler  but  more  stable  ones.  These  end 
products  are  carbon  dioxide,  water  and  urea,  together  with 
some  sulphates  and  phosphates,  the  production  of  which  is 
comparatively  immaterial.  The  urea  is  distinctive.  Heat, 
which  is  equivalent  to  so  much  energy,  is  evolved  in  the  oxi- 
dation process. 

It  is  probable  that  not  all  the  proteid,  under  the  ordinary 
diet,  is  actually  built  up  into  cell  substance.  A  part  of  it 
seems  to  be  destroyed  without  being  transformed  into  pro- 
toplasmic material,  but  the  destruction  always  takes  place 
through  the  agency  of  the  cells,  and  the  end  products  are  al- 
ways the  same  whether  disassimilation  of  the  proteid  occurs 
with  or  without  its  becoming  an  intrinsic  part  of  the  cell. 

Nitrogenous  Equilibrium — Circulating  and  Tissue  Pro- 
teids. — The  fact,  however,  that  the  characteristic  function  of 
the  nitrogenous  foods  is  to  furnish  protoplasmic  material 
should  not  be  lost  sight  of.  A  certain  amount  is  necessary 
to  maintain  "nitrogenous  equilibrium" ;  that  is,  to  keep  the 
intake  of  nitrogen  up  to  the  output.  When  nitrogenous  food 
is  withdrawn  there  continues  to  be  a  discharge  of  urea,  which 
is  the  chief  nitrogenous  excretion  and  the  amount  of  which 
represents  the  amount  of  nitrogenous  disassimilation  in  the 
body.  The  urea  eliminated  under  these  conditions  must  rep- 
resent the  actual  destruction  of  cell  substance,  and,  since  the 
supply  is  zero  and  the  output  is  considerable,  there  is  not  a 
state  of  nitrogenous  equilibrium;  the  animal  is  suffering 
destruction  of  its  protoplasm  without  a  compensatory  con- 
structive process.  On  the  other  hand,  the  supply  of  nitro- 
genous material  may  be,  and  usually  is,  in  excess  of  the  de- 
mands of  the  cells  for  the  actual  regeneration  of  their  sub- 
stance. This  excess  may  be  termed  "circulating  proteid," 
and  is  that  just  referred  to  as  being  oxidized  under  the  in- 
fluence of  the  cells,  but  without  being  transformed  into  pro- 
toplasm. That  part  of  the  nitrogenous  supply  which  is 
built  up  into  a  part  of  the  cell  has  been  called  "tissue  pro- 
teid" Whether  any  given  molecule  of  proteid  food  pass 


FOODS  IN  NUTRITION  175 

through  the  system  as  circulating  or  tissue  proteid  is  only  an 
accident — provided  the  supply  be  above  the  demand  of  the 
cells  for  tissue  proteid;  these  demands  are  the  first  to  be 
supplied  by  the  nitrogenous  material  at  hand. 

From  this  it  is  not  to  be  inferred  that  the  exigencies  of  nu- 
trition will  be  met  as  well  without  as  with  circulating  pro- 
teid. When  the  diet  consists  of  just  enough  proteid  to 
supply  the  tissue  wastes  and  of  ample  carbohydrate  and  hy- 
drocarbon materials,  the  nutritive  process  is  impaired.  It 
seems  necessary  to  perfect  health  that  the  supply  of  nitro- 
genous food  be  sufficient  to  allow  for  the  oxidation  of  some 
of  it  as  circulating  proteid  in  a  manner  analogous  to  oxida- 
tion of  the  non-nitrogenized  materials.  Life  can  be  main- 
tained on  nitrogenous  food  alone,  but  it  is  obvious  that  when 
this  is  done  the  amount  of  circulating  proteid  must  be  enor- 
mously increased  so  that  it  may  be  oxidized  to  furnish  energy 
for  the  body;  for  those  substances,  the  oxidation  of  which 
corresponds  to  oxidation  of  circulating  proteids  and  which 
furnish  the  main  supply  of  energy  for  doing  work  (viz.,  the 
carbohydrates  and  hydrocarbons),  are  now  withdrawn  from 
the  economy.  It  follows,  conversely,  that  the  ingestion  of 
carbohydrates  and  hydrocarbons  lessens  the  amount  of  pro- 
teid necessary  to  nutrition. 

The  albuminoids,  such  as  gelatin  (not  meant  to  be  in- 
cluded under  the  term  "nitrogenous"  foods,  though  they  con- 
tain nitrogen),  cannot  take  the  place  of  tissue  proteid;  they 
may  be  burnt  in  lieu  of  the  circulating  proteids  and  supply 
energy  just  as  the  carbohyrdates  and  fats  do. 

It  is  to  be  remembered  that  any  excess  of  proteid  or  al- 
buminoid food  is  not  discharged  as  such  in  the  excreta,  but 
undergoes  oxidation,  the  end  products  of  which  are  always 
the  same,  water,  carbon  dioxide  and  urea,  or  related  sub- 
stances ;  the  development  of  heat  is  also  an  invariable  accom- 
paniment of  their  destruction. 

While  a  person  may  live  on  proteid  food,  the  amount 
necessary  taxes  the  digestive  and  excretory  organs  to  such 


176  NUTRITION,   DIETETICS    AND  ANIMAL    HEAT 

an  extent  that  life  is  probably  shortened.  Since  the  total 
amount  of  urea  is  discharged  by  the  kidney,  that  organ,  un- 
der an  excess  of  proteid  diet,  is  particularly  prone  to  degen- 
erative changes  of  a  most  serious  nature. 

2.  The  carbohydrates  enter  the  blood  from  the  alimentary 
canal  as  dextrose,  are  conveyed  to  the  liver  and  converted 
into  glycogen,  which  is  stored  up  there  to  be  dealt  out  to  the 
blood  gradually,  after  being  reconverted  into  dextrose.  Dex- 
trose exists  in  the  blood  for  a  short  time  only,  being  con- 
verted into  other  substances,  but  its  final  oxidation  is  ef- 
fected by  the  tissues.  Its  end  products  are  carbon  dioxide 
and  water,  with  heat.  Sugar  (dextrose)  injected  into  the 
blood  soon  disappears.  It  is  thought  by  some  to  be  con- 
verted into  alcohol  in  the  blood  and  then  oxidized.  At  any 
rate,  the  formation  of  the  end  products  just  mentioned  is 
the  final  fate  of  the  carbohydrates,  through  whatever  split- 
ting processes  the  sugar  molecule  may  pass  before  it  is  con- 
verted into  these  substances. 

The  removal  of  the  pancreas  occasions  diabetes  mellitus, 
and  the  inference  is  that  this  gland  gives  off  to  the  blood 
some  internal  secretion  which  splits  up  the  sugar  molecule 
in  the  blood.  Hbw  this  lesion  causes  the  disease  in  question 
is  not  clear,  but  the  retention  of  a  small  part  of  the  gland 
enables  the  oxidation  of  sugar  by  the  tissues  to  proceed  in 
the  proper  way  and  it  is  not  discharged  in  the  urine. 

Value  of  the  Carbohydrates  in  Nutrition. — The  distinctive 
function  of  the  carbohydrates  is  to  act  as  fuel  for  the  body 
machine ;  they  are  burnt  up  to  supply  heat,  and  heat  repre- 
sents energy.  Hydrogen  and  oxygen  exist  already  in  the 
proportion  to  form  water — one  of  the  end  products — and 
only  enough  O  is  required  to  unite  with  the  carbon  of  the 
carbohydrates  to  form  CO2 — the  other  end  product.  The 
burning  (oxidation)  of  a  carbohydrate  outside  the  body  re- 
sults in  the  formation  of  CCte  and  H2O  and  the  elimination 
of  heat,  which  last,  if  properly  utilized,  can  be  converted  into 
energy — the  power  to  do  work.  The  result  of  the  oxidation 


FOODS  IN  NUTRITION  •  177 

of  a  carbohydrate  in  the  body  is  the  same.  Since  this  class 
of  food  is  easily  handled  by  the  alimentary  canal,  requires 
little  extra  O  for  its  destruction,  and  is  very  abundantly  sup- 
plied by  the  vegetable  world,  it  is  the  most  economical  from 
digestive,  absorptive,  respiratory  and  financial  standpoints. 
Carbohydrates  may  also  be  deposited  as  adipose  tissue  as 
will  be  seen  presently. 

3.  The  fats  have  the  same  general  office  in  nutrition  as  the 
carbohydrates,  viz.,  the  furnishing  of  energy  by  oxida- 
tion. They  leave  the  alimentary  canal  by  way  of  the  lacteals, 
are  conveyed  by  the  blood  to  the  tissues  and  there  oxidized 
with  the  formation  of  carbon  dioxide  and  water  and  the  lib- 
eration of  heat.  Though  more  O  is  necessary  to  burn  up  the 
fat  than  the  carbohydrate  molecule,  oxidation  of  the  fat  is 
attended  with  the  liberation  of  the  greater  amount  of  heat — 
i.  e.,  of  energy.  This  would  seem  to  indicate  that  it  would 
be  more  economical  to  eat  fats  to  the  exclusion  of  carbo- 
hydrates, since  a  smaller  quantity  of  the  former  will  supply 
the  requisite  amount  of  energy.  This  is  theoretically  true, 
but  considerations  of  digestion  render  it  not  practically 
so,  since  fats  tax  the  digestive  apparatus  much  more  than 
carbohydrates. 

The  fat  deposited  in  the  body — the  adipose  tissue — what- 
ever may  be  its  source,  it  is  to  be  looked  upon  as  so  much 
stored-up  energy.  When  the  supply  of  blood  is  cut  off  it  is 
the  first  part  of  the  organism  to  be  consumed.  Hence,  a  fat 
animal  will  survive  starvation  longer  than  a  lean  one. 

The  individuality,  the  functional  activity,  and  the  proper- 
ties involved  in  regeneration  of  protoplasm  are  ultimately 
dependent  upon  its  nitrogenous  characters.  The  other  con- 
stituents are  more  or  less  passive.  The  oxidation  of  fats  and 
carbohydrates,  however,  takes  place  under  the  influence  and 
through  the  agency  of  the  cells.  It  is  scarcely  necessary  to 
add  that  neither  fats  nor  carbohydrates,  nor  both  together, 
are  sufficient  to  sustain  life;  for  life  is  embodied  in  proto- 
plasm and  protoplasm  must  have  nitrogen,  which  element 
these  foods  cannot  furnish. 
12 


178  NUTRITION,    DIETETICS    AND   ANIMAL    HEAT 

Formation  of  Adipose  Tissue. — The  adipose  tissue  in  the 
body  is  not  the  result  of  direct  deposition  of  the  oleaginous 
foods.  The  amount  of  fat  taken  on  in  a  given  time  by  some 
animals,  as  hogs,  is  often  far  in  excess  of  the  quantity  of 
fat  in  the  ingesta.  Adipose  tissue  is,  under  normal  condi- 
tions, the  result  always  of  changes  due  to  protoplasmic  ac- 
tivity. It  is  formed  by  the  tissues  chiefly  from  the  carbohy- 
drates, but  also  to  a  less  extent  from  the  proteids.  The 
chemical  changes  by  which  sugar  is  converted  into  fat  are  as 
yet  undetermined,  but  there  are  so  many  evidences  of  an  in- 
crease in  body  fat  upon  an  excess  of  carbohydrate  food  that 
the  fact  itself  that  this  class  of  food  is  the  main  source  of 
fat  is  no  longer  disputed. 

As  regards  the  formation  of  fat  from  proteids,  it  is 
thought  that  the  molecule  is  split  up  into  a  nitrogenous  mole- 
cule, which  is  discharged  as  urea,  and  a  non-nitrogenous, 
which  at  once,  or  after  undergoing  other  changes,  is  depos- 
ited as  fat.  Experimental  observations  demonstrate  that  the 
liver  produces  gyycogen  on  a  purely  proteid  diet.  Since 
glycogen  is  a  carbohydrate,  and  carbohydrates  are  the  chief 
source  of  body  fat,  it  is  not  improbable  that  the  non-nitro- 
genous molecule  of  the  proteid  dissociation  takes  the  form 
of  glycogen  and  is  later  converted  into  fat  after  the  manner, 
whatever  it  may  be,  of  the  glycogen  introduced  in  carbohy- 
drate form.  When  the  carbon  discharged  is  less  than  the 
carbon  ingested  the  deficit  is  thought  to  be  retained  to  form 
fat,  which  is  deposited  as  a  reserve  to  be  used  whenever  its 
oxidation  may  become  necessary  as  a  supply  of  energy. 

It  follows  that  to  reduce  body  fat  the  carbohydrates  should 
be  largely  interdicted,  while  to  increase  it  they  should  be 
taken  in  excess.  In  human  beings  proper  regulation  of  the 
diet  is  more  efficacious  in  reducing  than  increasing  the 
amount  of  adipose  tissue. 

Adipose  Tissue  a  Reserve  Supply  of  Energy. — The  carbo- 
hydrates and  fats  are  preeminently  the  energy-producing 
foods,  and  of  these  the  carbohydrates,  for  reasons  indicated, 


CONDITIONS  INFLUENCING  METABOLISM  1/9 

are  the  more  important.  They  not  only  furnish  energy 
which  is  immediately  used  up  in  running  the  machinery  of 
the  body,  but  they  deposit,  or  attempt  to  deposit,  a  reserve 
supply  to  protect  the  proteid  portions  of  the  organism 
against  accidents  to  temporary  deprivation  of  food,  demands 
for  an  unusual  amount  of  energy,  malnutrition  from  vari- 
ous causes,  etc. — savings  laid  by  for  the  proverbial  rainy  day. 
This  reserve  supply  takes  the  form  first  of  glycogen,  which 
is  soon  used  up,  meeting  as  it  were  only  the  demands  of  the 
hour,  and  second  of  fat,  which  begins  to  be  drawn  upon  when 
the  glycogen  is  exhausted,  and  which  lasts  for  a  length  of 
time  depending  upon  its  amount. 

Conditions  Influencing  Metabolism. — Regular  exercise  is 
undoubtedly  favorable  to  the  nutrition  of  any  part,  as  e.  g., 
the  muscles,  the  brain,  etc.  Exercise  may  mean  increased  dis- 
assimilation,  but  if  so  it  also  means  increased  assimilation. 
With  regard  to  muscular  exercise  of  average  severity  and 
reasonable  duration,  the  results  of  cellular  activity  seem  at 
first  a  little  surprising,  but  are  really  to  be  expected  if  the 
concluding  remarks  of  the  previous  paragraph  are  true.  The 
amount  of  urea  under  such  exercise  is  not  appreciably  in- 
creased— which  means  that  disassimilation  in  the  protoplasm 
of  the  muscle  cells  is  not  increased.  This  remark  holds  good 
however,  only  when  the  supply  of  sugars,  starches  and  fats 
is  abundant ;  if  they  are  not  present  in  sufficient  quantity  to 
meet  the  increased  demand  for  energy-supplying  materials, 
then  the  proteids  must  be  oxidized  to  furnish  it,  and  the  urea 
discharged  is  increased.  In  striking  contrast  to  the  constant 
output  of  urea  is  the  largely  increased  output  of  CO2,  repre- 
senting oxidation  of  the  carbohydrates  and  fats. 

During  sleep  the  nitrogenous  output  is  not  materially  di- 
minished, while  that  of  CO2  is  markedly  less.  This  is  ex- 
plained by  the  fact  that  there  is  less  energy  needed  and  cor- 
respondingly less  oxidation  of  the  energy-producing  mate- 
rials. Proteid  metabolism  is  undisturbed. 
f  A  low  external  temperature  does  not  increase  the  output 


I-80  NUTRITION,   DIETETICS   AND  ANIMAL    HEAT 

of  urea ;  it  increases  the  output  of  CCte.  These  two  facts  to- 
gether mean  again  that  only  the  carbohydrates  and  fats  are 
being  oxidized  in  increased  amount.  This  increased  oxida- 
tion, the  effect  of  which  is  to  maintain  the  normal  body  tem- 
perature is  usually  dismissed  with  the  statement  that  it  is  a 
reflex  nervous  act.  It  is  claimed  by  Johannson  that  the  CO 
output  is  not  increased  until  shivering  occurs  (Reichert). 
That  being  the  case,  the  increase  is  explained  on  the  ground 
of  increased  energy  and  heat  production  incident  to  muscular 
exercise,  and  shivering  assumes  the  dignity  of  a  physiological 
factor  in  keeping  up  the  temperature  of  the  body.  This  is 
perfectly  reasonable  when  it  is  remembered  how  effective  ac- 
tive muscular  exercise  is  in  keeping  the  body  warm.  But  the 
fact  that  a  person  when  cold  shivers  and  is  restless  involun- 
tarily does  not  allow  us  to  escape  the  unsatisfactory  "reflex 
action"  explanation  of  the  phenomenon  in  question.  Within 
ordinary  and  reasonable  limits  proteid  metabolism  is  undis- 
turbed; it  is  still  being  protected  by  the  fats  and  carbohy- 
drates. 

During  starvation  nothing  is  supplied  from  the  outside 
world  except  oxygen,  and  the  animal  must  live  on  the  mater- 
ials already  in  his  body.  The  glycogen  is  first  consumed ;  it  is 
the  surplus  on  hand ;  but  at  best  it  is  all  gone  in  a  very  few 
days.  Then  the  fat  stored  up  as  adipose  tissue  is  drawn 
upon ;  it  is  the  reserve  fund ;  but  it  is  likewise  soon  con- 
sumed; the  animal  becomes  progressively  emaciated.  When 
this  is  exhausted  the  tissue  proteid  is  attacked;  this  is  the 
capital  and  is  the  last  to  be  touched ;  but  there  must  be  heat 
and  at  least  some  energy,  and  there  is  no  other  source.  When 
the  proteid  capital  has  at  least  been  so  impaired  that  it  can  no 
longer  furnish  heat  to  maintain  the  body  temperature  and 
energy  to  carry  on  the  necessary  organic  functions,  the  or- 
ganism is  physiologically  bankrupt  and  assignment  follows 
— death  is  at  hand. 


REQUISITES   OF   DIET  l8l 

DIETETICS. 

The  appetite,  under  normal  conditions,  may  be  depended 
upon  to  regulate  both  quantity  and  quality  of  diet  in  a  fairly 
satisfactory  manner.  Different  peoples  require  different  pro- 
portions and  amounts  of  the  various  food  stuffs  and  the 
same  is  true  of  any  given  individual  for  varying  conditions 
of  temperature,  exercise,  etc.  But  in  any  case  the  object  of 
eating  is  to  prevent  the  loss,  in  aggregate,  of  proteid  tissue, 
fat,  etc. — to  replace  the  wastes,  and  that  in  the  most  conveni- 
ent and  economical  way. 

When  the  ingesta  exceed  the  excreta  the  animal  is  gaining 
in  weight ;  when  opposite  conditions  obtain  he  is  losing ;  when 
there  is  a  balance  between  the  two  the  body  equilibrium  is 
being  maintained. 

Determination  of  the  Requisites  of  a  Diet. — The  usual 
method  of  determining,  in  a  scientific  manner,  the  requisites 
of  a  normal  diet  for  persons  in  general  is  to  estimate  the 
amount  of  the  various  excretions  from  the  bodies  of  a  lim- 
ited number  of  persons  in  health,  and  from  this  knowledge 
to  calculate  the  amount  and  kind  of  food  which  will  supply 
the  demands  in  the  most  satisfactory  way,  it  being  assumed 
that  these  excretions  represent  the  normal  and  necessary 
metabolism  going  on  in  the  body.  The  results  of  such  ex- 
amination are  found  to  correspond  with  the  actual  demands 
of  the  system. 

It  has  been  seen  that  the  organism  demands  some  fifteen 
or  more  chemical  elements  for  use  to  keep  itself  in  good  run- 
ning order ;  it  has  been  seen  also  that  its  demands,  so  far  as 
quantity  is  concerned,  are  chiefly  confined  to  carbon,  hydro- 
gen, oxygen  and  nitrogen.  The  other  elements  deserve  no 
attention  here  since  they  (excepting  sodium  chloride)  are 
unconsciously  introduced  with  the  ordinary  foods  in  amounts 
sufficient  to  satisfy  the  requirements  of  the  system.  More- 
over, the  air  we  breathe  and  the  water  we  drink  furnish  an 
ample  supply  of  hydrogen  and  oxygen  when  to  this  supply  is 


l82  NUTRITION,   DIETETICS   AND   ANIMAL    HEAT 

added  the  quota  of  these  elements  contained  in  the  necessary 
quantities  of  other  aliments.  So,  therefore,  if  we  fix  upon 
a  diet  which  will  furnish  the  requisite  amounts  of  carbon  and 
nitrogen  no  attention  need  t^e  paid  to  the  other  elements. 
The  supply  of  the  others  may  be  said  to  regulate  itself  if  the 
supply  of  carbon  and  nitrogen  be  regulated. 

The  object,  then,  of  food  may  be  said  to  be  the  replace- 
ment of  carbon  and  nitrogen — the  carbon  and  nitrogen  in  the 
excreta.  Of  these  two  elements,  carbohydrates  and  fats 
will  furnish  only  carbon ;  proteid  food  will  furnish  both. 

Amount  of  C  and  N  Necessary. — It  is  found  that  the  daily 
discharge  of  nitrogen  is  about  18  grams  (4^5)  and  of  car- 
bon about  281  grams  (8^2§).  These  are  the  amounts,  there- 
fore, which  must  be  supplied  by  food.  We  may  accept,  as 
representing  the  proteid  molecule  in  general,  the  formula,  Ci2 
HmOifflNisS.  Then  it  is  evident  that  an  amount  of  proteid 
food  which  would  furnish  the  necessary  18  grams  of  nitro- 
gen would  furnish  only  72  grams  of  carbon — only  about  one- 
fourth  enough.  If,  now,  the  proteid  food  be  increased  to 
supply  281  grams  of  carbon,  the  system  will  have  to  handle 
four  times  as  much  nitrogen  as  it  needs ;  and  this  is  a  tax  to 
the  digestive  apparatus  and  the  excretory  organs,  particu- 
larly the  kidney — a  tax  which  is  rendered  unnecessary  by  the 
availability  of  the  carbohydrates  and  fats  as  food.  These 
contain  abundance  of  carbon,  and  it  is  far  better  to  eat  only 
enough  proteid  food  to  supply  the  18  grams  of  nitrogen,  and 
make  up  the  deficit  of  carbon  with  non-nitrogenized  articles 
of  diet.  One  can  supply  all  the  demands  by  eating  nitro- 
genous food  alone,  and  life  will  be  preserved  indefinitely 
perhaps,  but  the  prediction  would  be  warranted  that  in  such 
a  case  the  person  would  probably  die  prematurely — as  a  re- 
sult of  kidney  or  liver  disease. 

Articles  Which  Will  Supply  the  Necessary  Amounts  of 
C  and  N. — The  conclusion  (modified)  of  Moleschott  is  that 
the  average  man  needs  daily  about  120  grams  of  proteid,  90 
grams  of  fat,  and  320  grams  of  carbohydrate  food,  estimated 


REQUISITES  OF  DIET 


dry ;  and  that  with  this,  in  the  usual  state  in  which  such  food 
is  taken,  he  will  consume  unconsciously,  or  as  a  result  of 
craving,  some  30  grams  of  salt  and  2,800  grams  of  water. 
These  proportions  are  supposed  to  satisfy  the  demands  of 
the  system  in  an  economical  way.  The  estimates  of  Ranke 
vary  somewhat  from  this  as  indicated  in  the  subjoined  table 
which  shows  also  the  balance  kept  up  in  the  body. 


Income. 

Expenditure. 

Foods. 

Nitrogen. 

Carbon. 

Excretions. 

Nitrogen. 

Carbon. 

Proteid,  loogm. 
Fat,  loogm. 
Carbohydrates, 
250  gm. 

iSogm. 
o.o   " 
o.o   " 

53-0  gm. 
79.0    " 
93-0    ' 

Urea,  31.5  gm. 
Uric  acid,  0.5 
gm. 
Feces 
Respiration 
(C02) 

1  14.4- 

i.i 
o.o 

6.16 

10.84 
208.00 

15-5  gm. 

225.0  gm. 

15-5 

225.00 

The  actual  amounts  of  given  substances  which  it  is  neces- 
sary to  eat  in  order  to  supply  the  requirements  of  these  esti- 
mates depend,  of  course,  on  the  composition  of  those  sub- 
stances, and  would  have  to  be  settled  by  reference  to  a  table 
giving  analyses  of  the  common  articles  of  diet.  Two  pounds 
of  bread  and  3/4  pound  (when  uncooked)  of  lean  meat,  to- 
gether with  water  and  salt,  will  supply  the  demands ;  but  this 
is  an  unusual  diet.  Or  i  pound  of  meat,  I  pound  of  bread 
and  l/4  pound  of  butter,  or  other  fat,  with  water  and  salt  is 
probably  preferable. 

In  any  case  if  nutrition  is  to  be  properly  performed  the 
diet  must  be  varied.  It  could  not  be  held  that  the  above 
supply  of  food  would  keep  a  person  indefinitely  in  good 
health,  His  demands  for  nitrogen  and  carbon  are  always 


184  NUTRITION,   DIETETICS   AND   ANIMAL    HEAT 

v  approximately  the  same,  but  the  organism  revolts  at  being 
supplied  with  them  from  exactly  the  same  source  for  any 
considerable  length  of  time. 

As  a  diet  is  necessary  (Schenck  and  Gurber)  : 

Proteid.  Fat.  Carbohydrates. 

Resting  man  100  gm.  60  gm.  400  gm. 

Resting  woman   go  gm.  40  gm.  350  gm. 

Working  man 130  gm.  100  gm.  500  gm. 

It  need  scarcely  be  added  that  any  condition,  such  as  exer- 
cise, temperature,  etc.,  which  increases  the  excreta,  calls  for 
a  larger  supply  of  ingesta.  Ordinary  exercise  is  allowed 
for  in  the  estimates  just  given. 

ANIMAL  HEAT. 

The  Temperature. — The  average  temperature  of  the  hu- 
man body,  taken  under  the  tongue,  is  98.5°  F.  It  varies  in 
different  parts,  the  mean  being  about  100°.  The  metabolic 
activity  in  different  parts  of  the  body  is  changeable,  and  con- 
sequently the  heat  production  in  all  parts  is  not  the  same. 

The  fact  that  -the  temperature  is  nearly  identical  through- 
out the  body  is  due  to  the  distribution  of  heat,  which  distri- 
bution is  mainly  effected  through  the  agency  of  the  circulat- 
ing fluids.  The  rectal  temperature  is  a  little  higher  than  that 
obtained  in  the  mouth.  The  temperature  of  arterial  is  higher 
than  that  of  venous  blood.  The  warmest  blood  is  in  the  hepat- 
ic veins ;  the  coolest  is  that  which  has  just  passed  through  the 
most  exposed  peripheral  parts,  as  the  helix  of  the  ear.  The 
mean  body  temperature  is  a  little  lower  in  the  morning  than 
in  the  evening,  in  the  female  than  in  the  male,  on  a  restricted 
than  on  an  abundant  diet,  in  cold  than  in  hot  climates,  and, 
in  general,  in  conditions  of  diminished  than  of  exalted  met- 
abolic activity. 

But  in  health  these  variations  are  of  trivial  importance  and 
do  not  represent  a  sweep  of  more  than  2°  F.  The  body  tern- 


HEAT  AND  FORCE  185 

perature  may  be  looked  upon  as  being  a  fairly  constant 
quantity.  It  varies  scarcely  at  all  with  variations  of  exter- 
nal temperature,  so  long  as  the  heat-regulating  apparatus  is 
in  order.  An  external  (dry)  temperature  of  212°  F.,  or  the 
extremely  low  temperature  of  some  regions,  can  be  borne 
with  very  slight  fluctuations  in  that  temperature  of  the  body. 
The  actual  limits  of  internal  temperature  consistent  with  the 
preservation  of  life  are  given  by  Flint  as  83°  and  107°  F. 
These  temperatures  cannot  be  long  endured. 

The  fundamental  fact  to  be  kept  constantly  in  mind  is 
that  there  is  a  continual  production  and  a  continual  dissipa- 
tion of  heat,  in  ways  to  be  indicated  presently.  These  two 
processes  are  known  as  thermogenesis  (heat  production)  and 
thermolysis  (heat  loss)  respectively.  The  preservation  of 
the  proper  balance  between  heat  production  and  heat  dissipa- 
tion is  known  as  thermotaxis. 

Supply  of  Heat  and  its  Relation  to  Force. — It  is  a  matter 
of  common  observation  that  the  burning  (oxidation)  of 
any  substance,  as  a  piece  of  wood  or  an  article  of  diet,  is  ac- 
companied by  the  evolution  of  heat.  It  is  also  known  that 
heat  may  be  converted  into  force — may  be  made  to  do  work. 
The  burning  of  a  fat  or  a  sugar  produces  CO2  and  H2O ;  the 
burning  of  a  proteid  produces  CCte  and  H2O,  and  additional 
substances.  The  final  products,  and  the  amount  of  heat 
evolved,  are  precisely  the  same  whether  the  oxidation  be 
rapid  or  slow.  Now,  the  oxidation  of  food  is  exactly  what 
occurs  in  the  human  organism,  though  that  of  the  proteids  is 
not  completely  effected ;  CO2  and  H2O  are  produced  from 
them,  and  the  "additional  substances"  mentioned  are  repre- 
sented by  urea.  This  process,  then,  is  the  source  of  body 
heat.  To  the  supply  thus  furnished  may  be  added  a  little 
from  reactions  between  non-energy  producing  materials  in 
the  body,  from  warm  foods  and  drinks,  and  from  friction  in 
the  vessels,  joints,  etc. 

The  foods  thus  possess  a  certain  potential  energy,  an  en- 
ergy which  may  be  converted  directly  or  indirectly  into  heat, 


l86  NUTRITION,   DIETETICS   AND   ANIMAL    HEAT 

or  its  equivalent.  The  potential  energy  of  the  foods  keeps 
up  the  body  temperature  and  supplies  force  for  doing  work. 
It  is  converted  into  heat  and  kinetic  energy.  Kinetic  energy 
is  working  energy,  and  is  represented  in  the  body  chiefly 
by  muscular  contractions.  But,  since  this  kinetic  energy  has 
its  source  in  the  transformation  of  food  stuffs,  and  since 
kinetic  energy  and  heat  are  mutually  convertible,  it  may  be 
assumed  that  all  the  potential  energy  of  the  foods  is  con- 
verted into  heat.  The  kinetic  energy  may  be  taken  as  rep- 
resenting so  much  heat,  and  the  total  production  of  heat 
(including  kinetic  energy)  as  representing  the  total  produc- 
tion of  energy.  Or,  to  state  the  case  differently,  the  potential 
energy  of  the  food  is  converted  into  heat,  a  part  of  which  ap- 
pears as  kinetic  energy.  .By  far  the  largest  part  of  this  po- 
tential energy,  however,  is  converted  directly  into  heat.  Not 
more  than  one-fifth  of  the  heat  produced  in  the  body  can  be 
utilized  to  do  work,  and  a  part  of  that  work  is  actually  con- 
verted indirectly  into  heat,  and  contributes  to  the  total  heat 
of  the  body,  by  overcoming  friction  incident  to  respiration, 
circulation,  movements  of  the  joints,  muscles,  etc. 

Potential  Value  of  Foods. — It  is  estimated  that  the  oxida- 
tion in  the  body  of  one  gram  of  fat  produces  9,300  calories 
of  heat,  one  gram  of  carbohydrate  4,100  calories,  and  one 
gram  of  proteid  4,100  calories.  These  figures  represent  the 
potential  energy  of  the  several  foods.  Fats,  it  is  seen,  pro- 
duce, weight  for  weight,  more  than  twice  as  much  energy  as 
other  foods,  but  reasons  have  been  given  why  they  cannot  be 
used  exclusively. 

A  calorie  is  the  amount  of  heat  necessary  to  raise  i  Kg 
of  water  from  o°  to  i°  C.  A  grammeter  is  the  amount  of 
energy  necessary  to  raise  I  gram  i  meter.  Now  since  heat 
and  work  are  only  different  forms  of  energy,  these  two  units 
— calorie  and  grammeter — have  each  equivalents  in  terms  of 
the  other.  One  calorie  equals  424.5  grammeters ;  'that  is,  the 
force  represented  by  one  calorie  will  raise  one  gram  424.5 
meters.  The  terms  kilo-calorie,  or  kilogramdegree,  and  kilo- 


TOTAL   AND   SPECIFIC    HEAT  l/ 

grammeter  are  used  sometimes,  and  represent  1,000  times 
the  calorie  and  grammeter  respectively. 

Total  and  Specific  Heat. — The  temperature  of  a  body  in- 
dicates nothing  as  to  the  quantity  of  the  heat  it  contains. 
The  degree  of  heat  requires  only  a  thermometer  to  deter- 
mine it,  but  the  quantity  depends  on  the  temperature,  the 
weight  and  the  specific  heat  of  the  substance  in  question. 

Specific  heat  is  analogous  to  specific  gravity.  Water  is 
taken  as  the  standard  in  both  cases.  If  it  require  only  .5  ca- 
lorie to  raise  I  gram  of  a  certain  substance  I  degree  C.,  the 
specific  heat  of  that  substance  is  said  to  be  .5.  The  specific 
heat  of  the  body  is  .8 ;  that  is,  whereas  it  requires  a  certain 
amount  of  heat  to  raise  150  pounds  of  water  to  a  certain 
temperature,  it  would  require  only  .8  as  much  to  raise  a  hu- 
man body  weighing  the  same  to  the  same  temperature.  To 
find  the  total  heat  in  calories  in  any  body  it  is  only  necessary 
to  multiply  the  weight  (in  grams)  by  the  specific  heat  and 
by  the  temperature  C.  Estimates  made  by  calorimetry  from 
these  data  and  from  the  potential  value  of  the  different  foods 
give  the  total  daily  heat  production  as  about  2,500,000  ca- 
lories for  the  average  individual.  This  is  equal  to  about  i,- 
400  calories  per  hour  per  kilo  weight. 

The  English  heat  unit  is  the  pound-degree  F.  It  is  the 
amount  of  heat  necessary  to  raise  I  pound  of  water  i  degree 
F.  Its  mechanical  equivalent  is  the  force  necessary  to  raise 
i  pound  772  feet.  The  estimates  just  given  in  the  metric 
system  when  translated  to  English  nomenclature  give  the  to- 
tal heat  production  for  24  hours  as  about  8,400  pound- 
degrees,  or  2.5  per  hour  per  pound  weight.  These  figures  are 
given  as  only  approximate  and  are  subject  to  change  by  many 
causes,  such  as  sex,  cardiac  and  respiratory  activity,  internal 
and  external  temperature,  exercise,  digestion,  age,  nervous 
influences,  the  body  weight,  etc. 

Thermogenesis. — Thermogenesis,  or  the  production  of 
heat,  is  the  result  of  activity  on  the  part  of  the  tissues,  nerves 
and  centers.  Now,  the  potential  energy  of  the  food  stuffs  is 


l88  NUTRITION,    DIETETICS   AND  ANIMAL    HEAT 

the  ultimate  source  of  all  bodily  heat  no  matter  how  it  may 
be  manifested,  and  it  is  evident  from  what  has  been  said  al- 
ready that  all  the  tissues  of  the  body  are  heat-producing  tis- 
sues, because  oxidation  processes  go  on  in  them  all.  But 
muscular  tissue  seems  to  be  endowed  with  special  heat-pro- 
ducing capabilities,  so  much  so  that  it  is  said  to  generate 
heat  as  a  specific  product,  and  not  as  a  mere  incident  of  its 
metabolism.  Muscle  will  reproduce  heat  when  entirely  at 
rest — when  the  nutritive  metabolic  changes  are  practically 
nothing.  The  process  seems  to  be  regulated  in  accordance 
with  the  needs  of  the  economy  by  means  of  a  nervous  me- 
chanism, making  the  production  of  heat  analogous  to  secre- 
tion. Separation  of  a  muscle  from  its  nerve  does  not  stop 
thermogenesis,  but  markedly  interferes  with  it  in  that  part. 
The  existence  of  distinct  thermogenic  nerves  has  not  been 
demonstrated.  The  existence  of  specific  thermogenic  centers 
seems  certain.  Some  of  them  increase  and  some  decrease 
thermogenesis. 

The  general  thermogenic  centers  are  in  the  spinal  cord. 
Centers  increasing  thermogenesis  are  probably  in  the  cau- 
date nuclei  of  the  corpora  stria,  the  optic  thalami,  pons  and 
medulla.  Irritation  of  these  regions  causes  a  rise  in  temper- 
ature. The  location  of  the  thermo-inhibitory  centers  is  a 
matter  of  speculation.  The  general  thermogenic  centers  in 
the  cord  probably  maintain  a  fairly  constant  pro- 
duction of  heat  independently,  but  they  are  subservient  to 
encephalic  centers  which  excite  them  to  increased  or  de- 
creased activity  by  reason  of  certain  impressions,  cutaneous 
or  otherwise,  which  they  have  received. 

Heat  Loss. — About  85  per  cent,  of  animal  heat,  dis- 
charged as  such,  is  lost  by  radiation  and  evaporation  from 
the  skin;  about  12  per  cent,  is  dissipated  in  the  lungs  by 
evaporation  and  in  warming  the  inspired  air ;  the  remainder 
is  discharged  in  the  urine  and  feces  (disregarding  the  small 
amount  which  goes  to  warm  ingested  articles). 

Hbat  is  radiated  from  the  body  just  as  from  a  hot  stove. 


CONDITIONS  INFLUENCING  HEAT  DISSIPATION  189 

Radiation  is  affected  by  the  conductivity  of  the  surrounding 
medium.  For  instance,  in  media  of  water  and  air  of  the 
same  temperature  the  radiation  is  greater  in  water,  because 
it  is  a  better  conductor  of  heat. 

Evaporation  from  the  skin  is  of  very  great  importance  in 
increasing  heat  dissipation.  582  calories  of  heat  are  con- 
sumed when  one  gram  of  water  is  vaporized ;  and  when  this 
evaporation  takes  place  on  the  skin  the  heat  is  abstracted 
largely  from  the  body.  This  is  said  to  represent  nearly  15 
per  cent,  of  the  total  heat  dissipation.  Hence  the  value  of 
perspiring  in  hot  weather.  Evaporation  also  takes  place 
from  the  moist  surfaces  of  the  lungs  and,  moreover,  when 
as  is  usually  the  case,  the  inspired  air  is  cooler  than  the  lung 
structure,  a  certain  amount  of  heat  is  consumed  in  warming 
it. 

But  it  is  not  to  be  inferred  that  loss  of  heat  takes  place 
from  the  body  just  as  from  an  inanimate  object.  On  the 
other  hand,  it  is  intimately  connected  with  and  influenced  by 
circulation,  respiration,  secretion  and  other  functions.  When 
there  is  a  tendency  for  the  body  temperature  to  rise,  the  cir- 
culation becomes  more  active  and  sends  more  blood  to  the 
periphery  to  be  cooled;  respiration  is  augmented,  causing  a 
greater  abstraction  in  the  lungs ;  the  secretion  of  sweat,  for 
instance,  is  increased. 

There  may  be  distinct  centers  governing  the  loss  of  heat. 

Conditions  Influencing  Heat  Dissipation. — These  have 
been  suggested  in  a  previous  section.  Heat  dissipation  is 
greater  in  proportion  to  weight  in  small  than  in  large  ani- 
mals because  the  radiating  surface  is  relatively  larger.  It  is 
less  in  the  female  than  in  the  male  because  she  has,  as  a  rule, 
a  larger  proportion  of  subcutaneous  fat,  which  is  a  poor  con- 
ductor of  heat.  It  is  less  when  the  body  is  covered  with 
clothing  which  is  a  poor  conductor  of  heat  than  when  the 
covering  conducts  heat  readily.  It  is  increased  when  the 
internal  temperature  is  raised  and  when  the  external  temper- 
ature is  lowered.  Any  general  increase  in  vascular  or 


I9O  NUTRITION,   DIETETICS    AND   ANIMAL    HEAT 

respiratory  activity  increases  heat  dissipation  for  reasons  al- 
ready given.  When  the  external  temperature  is  high  and 
the  air  is  dry  evaporation  is  more  abundant,  and  conse- 
quently heat  dissipation  is  greater  than  when  the  air  is  al- 
ready impregnated  with  moisture.  Hence  the  oppressiveness 
of  the  high  external  temperature  with  high  humidity.  In 
fever  heat  dissipation  is  usually  increased,  but  to  a  less  de- 
gree than  the  production. 

Thermotaxis. — Thermo  taxis  is  the  regulation  of  heat 
production  and  heat  dissipation  so  that  the  temperature  of 
the  body  may  remain  the  same.  It  is  evident  that  there  is 
frequently  a  transient  increase  or  decrease  of  thermogen- 
etic  activity ;  unless  there  be  a  corresponding  change  in  ther- 
molytic  activity  the  temperature  will  be  disturbed. 

The  temperature  of  the  body  is  not  necessarily  raised 
when  heat  production  is  increased,  or  lowered  when  it  is  de- 
creased ;  for  heat  loss  may  be,  and  in  health  is,  correspond- 
ingly increased  or  diminished.  Conversely,  a  change  in  heat 
loss  does  not  necessarily  mean  an  opposite  change  in  the 
body  temperature.  Alterations  which  do  occur  in  the  tem- 
perature are  the  result  of  the  improper  regulation  of  the 
heat  at  hand.  For  instance,  fever  may  result  from  average 
heat  production  and  deficient  heat  loss ;  from  increased  heat 
production  and  heat  loss  when  the  latter  is  increased  less  than 
the  former;  from  diminished  heat  production  and  heat  loss 
when  the  latter  is  diminished  less  than  the  former,  etc.  A 
subnormal  temperature  is  caused  by  opposite  conditions. 
The  temperature  remains  constant  when  heat  production  and 
loss  are  normal,  or  when  they  are  increased  or  decreased 
correspondingly. 

Thermotactic  activity  is  the  result  of  changes  in  the  tem- 
perature of  the  blood,  or  of  cutaneous  impressions.  A  rise 
in  the  temperature  of  the  blood  excites  heat  loss,  as  indi- 
cated. A  cold  atmosphere  increases  heat  loss,  but  at  the 
same  time  it  makes  impressions  on  the  cutaneous  nerves 
which,  when  carried  to  the  centers,  excite  heat  production 


THERMOTAXIS  igi 

and  thus  compensation  is  established.  A  cold  bath  lowers 
the  temperature  because  heat  loss  is  increased  more  than 
heat  production.  There  is  increased  radiation  because  of 
the  relatively  increased  difference  in  the  temperature  of  the 
body  and  of  the  surrounding  medium.  On  the  other  hand, 
the  cold  contracts  the  capillaries,  diminishing  the  amount 
of  blood  exposed  to  the  cooling  influence  of  the  water  and 
decreasing  the  amount  of  sweat ;  but  these  influences  tend- 
ing to  inhibit  heat  loss  are  not  equal  to  those  augmenting 
it.  However,  in  health,  thermotaxis  prevents  the  disturb- 
ance of  the  balance  between  thermogenesis  and  thermolysis 
to  any  great  extent,  and  the  temperature  cannot  be  lowered 
very  much.  These  are  only  examples  of  the  reciprocal  reT 
lations  maintained  between  the  production  and  dissipation 
of  heat,  a  disturbance  of  which  relations  is  prevented  under 
normal  conditions  by  thermotaxis.  Any  change  in  one  pro- 
cess is  followed  at  once  by  a  compensatory  change  in  the 
other. 


CHAPTER  X. 
EXCRETION  BY  THE  KIDNEYS  AND  SKIN. 

EXCRETION  of  the  various  foods  after  they  have  dis- 
charged their  several  functions  in  the  body  is  effected 
mainly  by  the  kidneys,  skin,  lungs  and  alimentary  canal. 
The  excretory  action  of  the  last  two  named  is  considered 
under  Respiration  and  Digestion.  Attention  is  again  called 
to  the  fact  that  it  is  impossible  to  differentiate  strictly  be- 
tween a  secretory  and  excretory  fluid.  The  urine  is  as  typi- 
cal of  the  excretions  as  any  fluid  to  be  found.  But  it  will 
be  convenient  to  speak  of-  the  "secretion"  of  urine  when 
reference  is  made  to  the  act  of  separating  its  constituents 
from  the  blood. 

THE  KIDNEYS. 

Anatomy. — The  kidneys,  one  on  each  side  of  the  body, 
are  behind  the  peritoneum  in  the  lumbar  region.  The  right 
is  usually  a  little  lower  and  a  little  lighter  than  the  left.  The 
hilum  from  which  the  ureter  springs  looks  inward  and  for- 
ward. The  kidney,  as  found  behind  the  peritoneum,  is  cov- 
ered with  a  considerable  amount  of  fat,  but  the  substance 
proper  of  the  organ  is  closely  surrounded  by  a  somewhat  re- 
sistant fibrous  capsule  which  in  health  can  be  easily  stripped 
away.  At  the  hilum  the  capsule  is  continued  inward  to  line 
the  pelvis,  infundibula  and  calyces. 

The  kidney  belongs  to  the  class  of  compound  tubular 
glands.  If  it  be  cut  into  two  halves  by  an  incision  passing 
through  the  two  borders  (and,  therefore,  through  the  hilum) 
an  idea  of  its  gross  divisions  is  objtained.  The  renal  sub- 
stance is  seen  to  be  divided  into  an  outer  layer,  known  as  the 

192 


STRUCTURE  OF   THE   KIDNEY 


193 


cortical  substance,  and  an  inner,  or  pyramidal,  portion.  In- 
ternally the  incision  reveals  a  cavity  into  which  the  ureter 
opens.  This  is  the  pelvis. 

y 

2" 


FIG.  54. — Longitudinal  section  through  the  kidney,  the  pelvis  of  the 

kidney,  and  a  number  of  renal  calyces. 

(From  Brubaker,  after  Tyson.) 

A,  branch  of  the  renal  artery;  U,  ureter;  C,  renal  calyx;  i,  cortex;  i',  medul- 
lary rays;  i",  labyrinth,  or  cortex  proper;  2,  medulla;  2',  papillary  portion  of 
medulla,  or  medulla  proper;  2",  border  layer  of  the  medulla;  3,  3,  transverse 
section  through  the  axes  of  the  tubules  of  the  border  layer;  4,  fat  of  the  renal 
sinus;  5,  5,  arterial  branches;  *,  transversely  coursing  medulla  rays  in  column 
of  Bertin. 

Tracing  the  divisions  of  the  pelvis  toward  the  kidney  sub- 
13 


194 


EXCRETION    BY   THE   KIDNEYS   AND   SKIN 


stance,  it  is  found  to  be  continued  by  three  short  canals,  one 
toward  the  upper,  one  toward  the  lower  and  one  toward  the 
central  portion  of  the  organ.  These  are  the  three  infun- 
dibula.  Each  infundibulum,  passing  outward,  subdivides 


Cortex. 


Boundary  or 
1  ^marginal 
zone. 


^Papillary 

zone. 


FIG.  55-     • 

LSt  of  a  pyramid  of  Malpighi;  PF,  pyramids  of  Ferrein;  RA,  branch  of  renal 
artery  with  an  interlobular  artery;  RV,  lumen  of  a  renal  vein  receiving  an  in- 
terlobular  vein;  VR,  vasa  recta;  PA,  apex  of  a  renal  papilla;  b,  b,  embrace  the 
bases  of  the  lobules.  (Stirling.) 

into  two  or  three,  or  more,  short  cylinder-like  canals  which 
receive  the  apices  of  the  pyramids.  These  are  the  calyces, 
each  of  which  receives  the  apex  of  one  or  more  pyramids. 
The  urine  thus  escaping  from  the  pyramidal  tubules  passes 
in  succession  through  the  calyces,  infundibula,  pelvis,  and 
thence  into  the  ureter. 


STRUCTURE  OF  THE  KIDNEY  195 

The  cortical  substance  constitutes  the  outer  layer  of  the 
kidneys  and  is  about  %  inch  thick.  It  is  reddish  and  granu- 
lar in  appearance.  From  it  pass  in  between  the  Malpighian 
pyramids  columns  known  as  the  columns  of  Bertin.  The 
cortical  substance  contains  the  glomeruli  and  convoluted  tu- 
bules together  with  blood-vessels  and  lymphatics  supported 
by  connective  tissues. 

The  pyramidal  substance,  also  called  the  medullary  sub- 
stance, consists  of  a  number  of  pyramids,  about  12-15,  whose 
bases  look  outward  and  rest  on  the  cortical  substance  and 
whose  apices  look  inward  and  are  received  into  the  calyces. 
These  are  called  the  pyramids  of  Malpighi.  They  contain 
uriniferous  tubules,  vessels,  etc.,  supported  by  connective 
tissue.  It  will  be  seen  that  these  tubes  converge  and  join 
each  other  in  passing  from  the  base  to  the  apex  of  the  pyra- 
mid, so  that  the  very  large  number  entering  the  base  is  rep- 
resented by  only  10-25  at  the  apex.  Thus  it  is  that  the  Mal- 
pighian pyramid  is  divided  into  a  number  of  smaller  pyra- 
mids. These  latter  are  the  pyramids  of  Ferrein,  and  cor- 
respond in  number  to  the  number  of  tubes  radiating  from 
the  apex  of  the  larger  pyramid.  The  medullary  substance  is 
marked  by  striae  which  have  the  direction  of  tubules  and 
which  are  caused  by  them.  Its  consistence  is  firmer  and  its 
color  is  darker  than  that  of  the  cortical  substance. 

Malpighian  Bodies. — These  are  scattered  throughout  the 
cortical  substance,  and  are  M.oo-^50  inch  in  diameter.  They 
consist  of  a  bunch  of  capillaries  in  the  shape  of  a  ball,  the 
glomerulus,  surrounded  by  the  extremity,  or  rather  the  be- 
ginning, of  one  of  the  renal  tubules.  At  the  point  where  the 
tubule  joins  the  Malpighian  tuft  it  is  constricted;  running 
then  over  the  glomerulus  it  reaches  the  afferent  artery  and 
the  efferent  vein  on  the  opposite  side ;  when  it  has  reached 
these  vessels  it  is  reflected  over  the  whole  network  of  capil- 
laries so  that  really  the  tuft  is  outside  the  tube,  but  practic- 
ally it  is  covered  by  a  double  layer  of  the  tube  wall.  A  space, 
the  beginning  of  the  tubule,  is  left  between  these  two  layers 


196 


EXCRETION  BY  THE  KIDNEYS  AND  SKIN 


and  into  it  the  glomerular  secretion  passes.  The  outer 
layer  is  the  capsule  of  Bowman  (or  Miiller).  Both  layers 
consist  of  a  single  stratum  of  flattened  epithelial  cells ;  those 
of  the  inner  layer  are  applied  closely  to  the  glomerulus  and 


FIG.  56. — Transverse  section  of 
a  developing  Mialpighian  capsule 
and  tuft  (human)  X  300. 

From  a  fetus  at  about  the  fourth 
month;  a,  flattened  cells  growing  to 
form  the  capsule;  b,  more  rounded 
cells  continuous  with  the  above,  re- 
flected round  c,  and  finally  enveloping 
it;  c,  mass  of  embryonic  cells  which 
will  later  become  developed  into  blood- 
vessels. (Kirkes  after  W.  Pye.) 


FIG.  57.  —  Epithelial  elements 
of  a  Malpighian  capsule  and 
tuft. 

With  the  commencement  of  a  urinary 
tubule  showing  the  afferent  and  effer> 
ent  vessels;  a,  layer  of  flat  epithelium 
forming  the  capsule;  b,  similar  but 
rather  larger  epithelial  cells,  placed  in 
the  walls  of  the  tube;  c,  cells  covering 
the  vessels  of  the  capillary  tuft;  d, 
commencement  of  the  tubule,  some- 
what narrower  than  the  rest  of  it. 
(Kirkes  after  W.  Pye.) 


are  thought  to  be  very  important  in  secretion.  The  incom- 
ing artery  breaks  up  to  form  the  capillary  tuft;  the  corre- 
sponding outgoing  vein  has  a  smaller  caliber  than  the  artery. 
The  vein,  having  left  the  glomerulus,  breaks  up  into  a  sec- 
ondary network  around  the  convoluted  tubes.  This  arrange- 


STRUCTURE   OF    THE    KIDNEY  197 

ment  of  the  Malpighian  body  furnishes  a  most  favorable 
opportunity  for  the  passage  of  substances  out  of  the  blood 
current  into  the  beginning  of  the  tube. 

Uriniferous  Tubules. — These  begin  at  the  glomeruli  and 
end  at  the  apices  of  the  Malpighian  pyramids.  From  their 
tortuous  course  in  the  cortical  portion  they  are  there  called 
convoluted  tubules,  in  contradistinction  to  the  straight  tubes 
of  the  medullary  portion.  This,  however,  is  only  a  general  di- 
vision ;  further  divisions  are  to  be  noted. 

The  constricted  portion  of  the  tube  where  it  leaves  the 
glomerulus  is  the  (i)  neck;  passing  away  from  the  neck  the 
tubule  becomes  very  tortuous  and  is  known  as  the  (2)  pri- 
mary convoluted  tubule,  which,  having  run  for  a  variable 
distance,  becomes  narrow  near  the  base  of  the  pyramid,  and 
taking  a  comparatively  straight  course  downward  enters  the 
pyramid  under  the  name  of  the  (3)  descending  limb  of 
Henle's  loop ;  some  of  these  run  nearly  as  far  as  the  apex, 
but  most  of  them  near  the  base  or  middle  of  the  pyramid 
turn  upward  forming  thus  (4)  Henle's  loop  and  beginning 
the  (5)  ascending  limb  of  Henle's  loop;  the  tube  having  re- 
entered  the  cortical  substance  becomes  convoluted  again, 
(6)  secondary  convolution,  which,  by  a  less  tortuous  con- 
tinuation, the  (7)  intermediate  tube,  communicates  with  the 
collecting  tubules,  or  the  (8)  straight  tubes  of  Bellini;  these 
last  beginning  in  the  cortex,  and  receiving  in  their  course 
large  numbers  of  intermediate  tubes,  enter  the  base  of  the 
pyramid  and  run  in  a  nearly  straight  direction  toward  the 
apex.  About  100  of  these  straight  tubes  entering  at  the  base 
join  in  their  course  downward  until  at  the  apex  they  are 
represented  by  a  single  tube.  These  collections  constitute 
the  pyramids  of  Ferrein ;  there  are  about  12-18  pyramids  of 
Ferrein  to  each  Malpighian  pyramid,  and  as  many  tubal  ori- 
fices at  the  apex.  The  so-called  zigzag  and  spiral  tubules 
are  here  considered  parts  of  the  first  and  second  convoluted 
tubules.  (See  Fig.  58.) 

Before  they  reach  the  collecting  tubules  the  tubes  vary  in 


198 


EXCRETION  BY  THE  KIDNEYS  AND  SKIN 


FIG.  58. — A  diagram  of  the  sections  of  uriniferous  tubes. 

A,  cortex  limited  externally  by  the  capsule;  a,  subcapsular  layer  not  containing 
Malpighian  corpuscles;  a',  inner  stratum  of  cortex,  also  without  Malpighian  cap- 
sules; B,  boundary  layer;  C,  medullary  part  next  the  boundary  layer;  i,  Bow- 
man's capsule  of  Malpighian  corpuscle;  2,  neck  of  capsule;  3,  firsfl  convoluted 
tubule;  4,  spiral  tubule;  5,  descending  limb  of  Henle's  loop;  6,  the  loop  proper; 
7,  thick  part  of  the  ascending  limb;  8,  spiral  part  of  ascending  limb;  g,  narrow 
ascending  limb  in  the  medullary  ray;  10,  the  zigzag  tubule;  u,  the  second  con- 
voluted tubule;  12,  the  junctional  tubule;  13,  the  collecting  tubule  of  the  medul- 
lary ray;  14,  the  collecting  tube  of  the  boundary  layer;  15,  duct  of  Bellini. 
(Kirkes  after  Klein.) 


STRUCTURE  OF  THE  KIDNEY  IQ9 

diameter  from  M.WO  to  ^ooo  inch;  the  collecting  tubules  pro- 
gressively increase  in  diameter  from  %oo  to  ^oo  inch.  The 
cells  lining  the  convoluted  and  intermediate  tubules  are  in- 
clined to  the  pyramidal  shape.  Their  bases  present  the  ap- 
pearance of  fibers  at  right  angles  to  the  basement  membrane 
(hence  "rodded"  cells),  while  their  opposite  extremities  are 
granular.  The  tubes  of  Henle  are  lined  by  flattened  epi- 
thelium for  the  most  part. 

The  division  is  somewhat  arbitrary,  but  the  secreting  por- 
tion of  the  tubules  is  supposed  to  be  confined  to  the  cortical 
substance,  while  the  tubes  of  the  medullary  substance  only 
carry  away  the  fluid. 

Blood  Supply. — The  renal  artery,  having  entered  the  hi- 
lum,  divides  into  branches,  two  of  which  usually  enter  each 
column  of  Bertin.  Running  upward  in  these  columns  the 
branches  give  off  small  arterial  twigs  to  the  substance  of 
the  column.  When  a  point  opposite  the  bases  of  the  Malpig- 
hian  pyramids  is  reached  each  branch  follows  the  convex 
base  of  the  pyramid  to  which  it  is  adjacent,  the  one  branch 
going  in  an  opposite  direction  to  the  other.  Each  meets  a 
corresponding  branch  from  the  other  side  of  the  pyramid, 
and  thus  a  convex  arterial  arch  covers  the  base  of  the  pyra- 
mid from  which  arch  branches  go  inward  to  supply  the  me- 
dullary substance  and  outward  to  furnish  branches  to  the 
glomeruli.  The  arrangement  of  the  vessels  in  relation  to  the 
Malpighian  bodies  has  been  noticed.  In  the  glomerulus  the 
capillaries  do  not  form  a  true  anastomosis,  but  this  is  not 
true  of  the  network  surrounding  the  convoluted  tubes. 

Mechanism  of  Urinary  Secretion. — Histologists  have  been 
unable  to  demonstrate  the  presence  of  distinct  secretory 
fibers  for  the  glomerular  or  tubal  cells.  This  leaves  the  me- 
chanism of  secretion  to  be  explained  by  (i)  the  vascular 
supply  and  by  (2)  the  "vital  activity"  of  the  cells — both  op- 
erating in  conjunction  with  osmosis. 

Irritation  of  a  certain  part  of  the  floor  of  the  fourth  ven- 
tricle occasions  certain  marked  changes  in  the  quantity  and 


2OO 


EXCRETION  BY  THE  KIDNEYS  AND  SKIN 


quality  of  the  urine ;  secretion  of  the  upper  dorsal  cord  tem- 
porarily arrests  the  secretion;  mental  emotions,  such  as 
fright,  anxiety,  etc.,  also  modify  the  flow.  All  these  circum- 
stances, and  many  others,  indicate  some  control  over  the  ac- 


\ 
FIG.  59. — Blood-vessels  of  the  kidney. 

A,  capillaries  of  cortex;  B,  of  medulla;  a,  interlobular  artery;  i.  vas  afferens; 
2,  vas  efferens;  i,  e,  vasa  recta;  VV ,  interlobular  vein;  S,  origin  of  a  stellate 
vein;  i,  i,  Bowman's  capsule  and  glomerules;  P,  apex  of  papilla;  C,  capsule  of 
kidney;  e,  vasa  recta  from  lowest  vas  efferens.  (Stirling.) 


STRUCTURE  OF  THE-  KIDNEY  2OI 

tivity  of  the  kidneys  by  the  nervous  system;  but  that  influ- 
ence is  probably  exerted  only  through  vaso-constrictor  and 
vaso-dilator  fibers  to  the  vessels. 

Assuming  for  the  present  that  nearly  all  the  constituents 
of  urine  preexist  in  the  blood  and  are  simply  taken  out  of 
the  circulation  in  the  kidney,  it  may  be  stated  that,  for  the 
most  part,  the  water  and  salts  are  extracted  by  the  cells  of 
the  Malpighian  bodies,  while  the  urea  and  related  nitrogen- 
ous solids  are  removed  by  the  cells  of  the  convoluted  tubes ; 
so  that  the  specific  gravity  of  the  fluid  is  raised  in  passing 
down  the  tubes.  While  the  histology  of  the  kidney,  and 
especially  the  arrangement  of  the  glomeruli,  is  most  favor- 
able for  the  exercise  of  simple  osmosis,  and  while  this  pro- 
cess is  doubtless  mainly  responsible  for  the  phenomena 
which  occur,  it  seems  highly  probable  that  the  cells  them- 
selves modify  osmotic  action  by  taking  an  active  part  in  the 
secretion  of  urine.  They  undoubtedly  exercise  a  selective 
affinity  accounting  for  the  different  materials  handled  by  the 
glomeruli  and  the  tubes.  Moreover,  morphological  changes 
in  the  tubal  cells  during  activity  have  been  microscopically 
demonstrated.  Vesicles  are  described  as  forming  in  the  body 
of  the  cell,  approaching  the  lumen,  bursting  and  discharging 
their  contents — which  are  supposed  to  include  the  urea  and 
such  other  materials  as  may  be  here  extracted  from  the 
blood. 

As  regards  the  elimination  of  water  and  salts  by  the 
glomerular  epithelium,  it  must  also  be  admitted  that  the 
cells  take  some  obscure  but  active  part.  Were  this  only  an 
osmotic  process  the  amount  eliminated  would  vary  exactly 
as  the  pressure.  While  usually  a  rise  in  renal  blood-pressure 
is  accompanied  by  an  increased  flow  of  urine  and  a  fall  by 
a  correspondingly  decreased  flow,  the  rule  does  not  always 
hold  good.  For  instance,  compression  of  the  renal  vein 
raises  the  pressure  but  does  not  increase  the  amount  of 
urine. 

Another  fact,  which  seems  almost  if  not  quite  as  invari- 


2O2  EXCRETION  BY  THE  KIDNEYS  AND  SKIN 

able  as  the  effect  of  blood-pressure,  is  that  the  amount  of 
urine  varies  directly  as  the  amount  of  blood  passing  through 
the  kidney,  independently  of  the  pressure;  and  these  two 
facts  constitute  about  all  that  is  definitely  known  concerning 
the  local  conditions  affecting  the  amotmt  of  urine.  Whether 
diuretics  increase  the  urinary  flow  by  simply  drawing  water 
from  the  tissues  into  the  blood  and  thus  increasing  the 
amount  and  pressure,  or  by  stimulating  the  cells  of  the 
glomeruli  to  increased  functional  activity  is  a  matter  as  yet 
undetermined. 

Properties  and  Composition  of  Urine. — When  an  ordi- 
nary amount  of  liquid  is  ingested  and  when  the  skin  is 
moderately  active  the  urine,  in  normal  conditions,  has  a  clear 
reddish  amber  color  and  a  specific  gravity  of  about  1020. 
The  more  fluid  ingested  the  paler  will  be  the  color  and  the 
lower  the  specific  gravity ;  the  more  active  the  skin  the  higher 
will  be  the  color  and  specific  gravity.  The  urine  is  diluted 
in  the  first  case  and  concentrated  in  the  second.  The  fact  is, 
the  amount  of  solids  (represented  by  urea)  to  be  eliminated 
in  24  hours  remains  approximately  the  same,  and  those  solids 
will  cause  a  high  or  low  specific  gravity  according  as  little 
or  much  water  is  eliminated  with  them.  The  average  amount 
of  urine  for  a  day  is  2  or  3  pints.  Normally  it  has  an  acid 
reaction  from  the  presence,  not  of  a  free  acid,  but  of  acid 
salts — chiefly  acid  sodium  phosphate.  The  odor  is  not  dis- 
agreeable on  ejection,  but  decomposition  soon  begins  and  a 
characteristic  offensive,  ammoniacal  odor  develops. 

The  kidney  is  the  most  important  excretory  organ  in  the 
body  and  the  large  number  of  urinary  constituents  is  not  sur- 
prising. The  chief  organic  constituents  are  urea,  uric  acid, 
hippuric  acid,  xanthin,  hypoxanthin,  creatinin,  phenol,  indi- 
can,  oxalic  acid,  lactates,  etc.  The  phosphates,  nitrates, 
sodium  chloride,  and  carbon  dioxide  are  the  chief  inorganic 
materials. 

Urea  is  the  most  important  of  the  nitrogenous  constitu- 
ents. It  contains  a  large  amount  of  nitrogen.  Nearly  all 


FORMATION  OF  UREA  2O3 

of  it  is  removed  from  the  body  by  the  kidneys,  and  double 
nephrectomy  means  death  from  its  retention.  Its  formation 
is  constant  and  its  removal  necessary.  Its  presence  in  the 
blood  seems  to  be  the  normal  stimulus  exciting  the  activity 
of  the  cells  of  the  convoluted  tubes. 

Whether  urea  is  produced  directly  in  the  tissues,  or 
whether  only  certain  substances  antecedent  to  it  are  there 
formed,  it  cannot  be  doubted  that  it  is  the  chief  final  pro- 
duct of  nitrogenous  ingesta  and  nitrogenous  dissimilation. 
It  is  practically  the  only  way  in  which  the  nitrogen  of  pro- 
teid  foods  can  escape  from  the  body.  It  exists  not  only  in 
the  blood  but  in  the  lymph,  vitreous  humor,  sweat,  milk,  sa- 
liva, etc.  It  has  been  stated  that  the  taking  of  large  quanti- 
ties of  liquids  lowers  the  specific  gravity  of  the  urine  by  di- 
luting it ;  this  is  true,  but  the  actual  amount  of  urea  is  in- 
creased somewhat  by  such  a  procedure.  It  is  not  surprising 
that  the  quantity  of  urea  is  largely  increased  when  much 
nitrogenous  food  is  taken,  and  that  it  is  greatly  decreased 
by  an  exclusively  vegetable  diet.  Anything,  like  exercise, 
which  will  increase  actual  tissue  metabolism,  will  increase 
the  output  of  urea,  while  anything  retarding  tissue  metabol- 
ism, like  alcohol,  will  decrease  the  output.  The  average 
amount  of  urea  for  24  hours  is  350  to  450  grains. 

Formation  of  Urea. — Seeing  that  urea  is  the  typical  end 
product  of  the  physiological  oxidation  of  the  proteids,  it  be- 
comes of  interest  to  determine,  if  possible,  where  urea  for- 
mation takes  place.  It  is  known  that  the  liver  is  very  active 
in  producing  this  substance ;  but  it  is  not  alone  by  this  organ 
that  urea  is  formed.  At  the  present  time  the  prevailing 
opinion  is  that,  for  the  most  part,  the  proteids  under  destruc- 
tive metabolism  in  the  tissues  do  not  reach  the  urea  stage  of 
transformation,  but  are  converted  into  ammonia  compounds 
(which  differ  very  slightly  from  the  urea  in  chemical  compo- 
sition), and  these  compounds  are  conveyed  by  the  blood  to 
the  liver,  where  the  slight  change  necessary  to  make  them 
urea  is  effected  under  the  influence  of  this  organ.  Ammon- 


204  EXCRETION  BY  THE  KIDNEYS  AND  SKIN 

mm  carbamate  seems  the  typical  compound,  but  ammonium 
carbonate  and  others  are  probably  likewise  converted.  Ar- 
tificial circulation  of  these  compounds  through  the  liver  gives 
rise  to  urea;  removal  of  the  liver  increases  the  ammonia 
compounds  and  decreases  the  urea  in  the  urine ;  ammonia 
compounds  are  normally  very  much  more  abundant  in  the 
portal  blood  than  in  the  arterial,  but  when  the  liver  is  re- 
moved they  are  evenly  distributed  throughout  the  circula- 
tion, and  the  animal  dies  in  a  few  days  of  symptoms  which 
can  be  aggravated  by  administration  of  the  ammonia  com- 
pounds;— all  of  which  circumstances  go  to  show  that  it  is 
ammonia  compounds  which  the  tissues  produce,  and  that 
they  are  changed  to  urea  in  the  liver. 

Still,  removal  of  the  liver  does  not  suspend  entirely  the 
output  of  urea.  Consequently  this  substance  must  be  formed 
elsewhere,  but  by  what  organs  is  unknown.  It  is  not  impos- 
sible that  it  is  formed  to  some  extent  in  all  organs  where 
proteid  dissociation  is  progressing.  This  is  practically,  if 
not  really,  the  case  in  health  at  any  rate,  even  under  the 
theory  above  mentioned. 

It  is  to  be  noted  that  urea  is  not  fidl  oxidized;  it  can  be 
oxidized  outside  the  body.  Thus  the  heat-producing  capac- 
ity of  the  proteids  is  not  completely  utilized.  If  they  have 
been  broken  down  in  the  body  into  substances  simpler  than 
urea,  then  the  amount  of  heat  liberated  in  such  dissocia- 
tion is  consumed  in  building  up  the  urea  molecule  to  be  dis- 
charged. 

Uric  acid  is  combined  in  normal  urine  to  form  the  urates 
of  sodium,  potassium,  magnesium,  calcium  and  ammonium. 
The  urate  of  sodium  is  by  far  the  most  abundant  of  these, 
and,  besides  urate  of  potassium,  only  traces  of  the  others 
are  found.  Free  uric  acid  in  human  urine  is  pathological. 
The  urates,  like  urea,  come  ultimately  from  oxidation  of  the 
nitrogenous  constituents  of  the  body.  They  are  not  formed 
in  the  kidney,  but  pass  out  as  such  from  the  blood.  About 
9-14  gr.  are  discharged  daily.  The  amount  is  increased  in 
gout. 


HIPPURIG  ACID  2O5 

111  some  animals  uric  acid  takes  the  place  of  urea,  none  of 
the  latter  being  formed.  In  these  cases  it  is  manufactured 
by  the  liver  from  ammonia  compounds.  This  does  not,  how- 
ever, seem  to  be  the  origin  of  uric  acid  in  human  urine.  It 
has  been  looked  upon  as  unconverted  urea,  i.  e.,  as  a  product 
antecedent  to  urea ;  but  at  present  such  does  not  seem  to  be 
the  case.  A  theory  that  it  is  the  end  product  of  the  destruc- 
tion of  certain  materials  in  the  nuclei  of  cells  has  consider- 
able support. 

Hippuric  acid  exists  in  the  urine  as  hippurates.  It  differs 
from  most  of  the  other  urinary  constituents  in  being  formed 
in  the  kidney ;  it  does  not  preexist  in  the  blood.  The  daily 
output  of  this  substance  is  about  10  grains,  though  the 
amount  may  be  considerably  increased  on  a  vegetable  diet. 
The  benzoic  acid  of  vegetables  seems  to  be  synthesized  into 
hippuric.  In  proteid  dissimilation  some  benzoic  acid  may  be 
produced  and  eliminated  in  this  shape. 

The  various  lactates  are  not  formed  by  the  kidney,  but 
pass  unchanged  into  it  from  the  blood.  The  lactic  acid  from 
which  they  are  formed  probably  results  from  the  transfor- 
mation of  dextrose. 

Creatinin  is  normally  present  in  the  urine.  It  is  a  nitro- 
genous body  differing  from  creatin  only  by  a  molecule  of 
water.  It  is  eliminated  to  the  extent  of  about  15  grains  per 
day.  A  part  comes  from  proteid  destruction  in  the  body,  and 
another  part  is  said  to  come  directly,  without  metabolism, 
from  creatin  which  is  a  constituent  of  ordinary  meat.  It  is 
not  formed  in  the  kidney. 

Xanthin,  hypoxanthin,  etc.,  are  to  be  regarded  as  nitro- 
genous excreta  allied  to  uric  acid  and  resulting  in  some  way 
from  proteid  metabolism.  They  are  regarded  by  some  as 
having  the  same  probable  origin  as  uric  acid,  viz.,  the  disin- 
tegration of  cell  nuclei. 

The  non-nitrogenous  constituents  scarcely  deserve  separ- 
ate mention.  It  is  through  the  kidney  that  the  largest  variety 
of  these  materials  are  discharged.  Certain  of  these  are  con- 


206  EXCRETION  BY  THE  KIDNEYS  AND  SKIN 

stant,  but  the  wide  variety  of  such  materials  taken  into  the 
alimentary  canal  accounts  for  the  same  wide  variety  in  the 
urine.  The  proportion  of  inert  substances  in  the  blood  is 
approximately  constant — kept  so  by  the  removal  of  any 
excess  by  the  kidneys  chiefly. 

Sodium  Chloride  is  eliminated  thus  to  the  extent  of  about 
151  grains  daily.  The  sulphates  are  unimportant.  About 
25  grains  are  excreted  daily.  The  phosphates  are  more  im- 
portant, the  acid  sodium  phosphate  being  mainly  responsible 
for  the  acid  reaction  of  the  urine.  Nitrogen  and  carbon  di- 
oxide are  the  chief  gases  to  be  found.  The  color  of  urine  is 
due  to  a  substance,  urochrome,  which  is  probably  formed 
from  hemoglobin.  Some  mucus  from  the  bladder  is  also 
in  the  urine. 

Variation  in  Amount  and  Composition  of  Urine. — "Its 
constitution  is  varying  with  every  different  condition  of  nu- 
trition, with  exercise,  bodily  and  mental,  with  sleep,  age,  sex, 
diet,  respiratory  activity,  the  quantity  of  cutaneous  exhala- 
tion, and  indeed  with  every  condition  which  affects  any  part 
of  the  system.  There  is  no  fluid  in  the  body  that  presents 
such  a  variety  of  constituents  as  a  constant  condition,  but 
in  which  the  proportion  of  these  constituents  is  so  vari- 
able"  (Flint). 

Prolonged  bodily  exercise  will  increase  the  amount  of 
urea,  but  the  urine  is  generally  decreased  in  quantity  because 
perspiration  is  more  active.  The  young  child  discharges  rel- 
atively much  more  urea  and  urine  than  the  adult.  The  fe- 
male discharges  relatively  more  urine,  but  less  urea,  than  the 
male.  Digestion  increases  the  urinary  flow.  Climate  and 
season  act  chiefly  though  increasing  or  diminishing  cutaneous 
activity.  Emotions  of  various  kinds  may  give  rise  to  an 
abundant  flow  of  pale  urine. 

Discharge  of  Urine. — On  leaving  the  pelvis  of  the  kidney 
the  urine  enters  the  ureters  and  passes  through  them  to  the 
bladder,  whence  it  is  discharged  per  urethram. 

The  ureters  run,  one  from  each  kidney,  downward  and 


THE  BLADDER  2O7 

slightly  inward  behind  the  peritoneum,  a  distance  of  some  18 
inches  to  the  base  of  the  bladder.  In  the  female  the  cervix 
uteri  lies  between  the  two  ureters  just  before  they  enter  the 
bladder.  They  penetrate  the  bladder  wall  obliquely,  their 
course  therein  being  nearly  an  inch  long.  The  effect  of  this 
arrangement  is  that  distention  of  the  bladder  closes  the  open- 
ing more  closely  instead  of  causing  regurgitation  into  the 
ureter.  The  ureter  is  composed  of  three  coats.  The  outer 
is  fibrous,  the  middle  muscular,  and  the  internal  mucous. 

The  bladder  serves  as  a  reservoir  for  the  urine  until  such 
time  as  it  is  convenient  for  it  to  be  evacuated.  This  organ, 
when  empty,  lies  deep  in  the  pelvis  in  front  of  the  rectum 
in  the  male  and  of  the  uterus  in  the  female.  When  moder- 
ately distended  it  will  hold  about  a  pint,  has  an  ovoid  shape 
and  rises  to  the  brim  of  the  pelvis.  It  also  has  three  coats. 
The  outer  is  peritoneal,  and  covers  the  posterior  and  small 
parts  of  the  lateral  and  anterior  surfaces  only.  Its  lower 
limit  posteriorly  is  the  entrance  of  the  ureters.  The  middle 
layer  is  muscular.  The  fibers,  which  are  non-striped,  are 
disposed  in  three  sheets.  Their  contraction  compresses  the 
contents  from  all  directions.  Embracing  the  neck  (outlet) 
of  the  bladder  is  a  thick  band  of  plain  muscle  tissue  known 
as  the  sphincter  vesicce.  The  tonic  contraction  of  this  mus- 
cle prevents  the  continual  escape  of  urine.  The  inner  coat 
of  the  bladder  is  mucous.  It  is  rather  thick,  and  loosely  ad- 
herent to  the  subjacent  muscular  coat  except  over  the  corpus 
trigonum  where  it  is  closely  attached.  The  corpus  trigonum 
is  a  triangular  body  of  fibrous  tissue  just  underneath  the 
mucous  membrane;  its  apex  is  at  the  origin  of  the  urethra, 
and  its  other  angles  are  at  the  vesical  openings  of  the  uret- 
ers. 

Absorption  from  the  intact  mucous  membrane  of  the  blad- 
der takes  place  very  sparingly,  if  at  all.  Abrasions  of  the 
membrane  from  any  cause  allow  absorption  to  occur;  and 
this  fact  may  be  made  use  of  to  locate  lesions  giving  rise  to 
hematuria.  Iodide  of  potassium  injected  into  the  bladder 


2O8  EXCRETION  BY  THE  KIDNEYS  AND  SKIN 

can  be  detected  in  the  saliva  if  the  bladder  is  the  source  of 
the  blood. 

Micturition. — When  the  bladder  has  become  moderately 
full  the  desire  to  expel  its  contents  arises.  The  act  of  mic- 
turition involves  relaxation  of  the  sphincter  vesica  and 
contraction  of  the  muscular  walls  of  the  bladder  aided  by 
the  abdominal  muscles  and  those  of  the  urethra.  A  slight 
contraction  of  the  abdominal  muscles  compresses  the  blad- 
der; after  a  short  interval  the  sphincter  relaxes  and  allows 
the  stream  to  pass  out  through  the  urethra.  When  the  act 
has  been  begun  contraction  of  the  bladder  will  suffice  to 
nearly  empty  the  organ,  but  complete  evacuation  is  finally 
brought  about  by  a  series  of  convulsive  contractions  on  the 
part  of  the  muscles  of  the  abdomen. 

The  center  controlling  the  reflex  nervous  phenomena  of 
micturition  is  opposite  to  the  fourth  lumbar  vertebra  in  the 
spinal  cord. 

THE  SKIN. 

Functions. — The  functions  of  the  skin  from  a  physical 
standpoint  are  sufficiently  apparent.  It  furnishes  protection 
to  the  underlying  parts,  preserves  the  general  contour  of  the 
body,  affords  lodgment  for  afferent  nerve  terminations,  and 
thus  establishes  relations  between  ourselves  and  our  sur- 
roundings. As  an  organ  of  excretion  it  is  very  important, 
and  in  fact  essential  to  life.  While  various  materials,  such 
as  urea  and  CO,  are  thus  discharged  from  the  body,  their 
amount  is  more  or  less  inconsequential,  and  it  appears  that  it 
is  the  action  of  the  skin  as  a  regulator  of  heat  "excretion" 
which  is  vital.  It  furnishes  one  of  the  three  chief  routes 
for  the  discharge  of  water  from  the  body,  and  it  will  be 
seen  that  it  is  largely  through  the  output  of  water  that  the 
output  of  heat  is  regulated.  So  necessary  is  the  skin  in  this 
respect  that  the  covering  with  impermeable  substances  of  as 
much  as  half  the  body  surface  is  followed  by  death. 

The  skin  excretions  are  contained  in  the  products  of  the 


STRUCTURE  OF  THE  SKIN 


2O9 


sebaceous  and  sweat  glands.  These  products  correspond  al- 
together to  neither  the  secretions  nor  the  excretions,  and  the 
sebaceous  glands  have  been  described  under  the  head  of 
secretion.  It  is  to  be  remembered,  however,  that  the  sweat 
usually  represents  part  of  the  sebaceous  as  well  as  the  sudo- 


Stratum  corneum. 

Stratum  lucidum. 
Stratum  granulosum. 


Stratum  Malpighii. 


FIG.  60. — Vertical  section  of  the  human  epidermis. 

The  nerve-fibrils,  n,  b,  stained  with  gold  chloride.      (Landois.) 

riparous  secretion,  because  the  mixture  of  the  two  is  a 
physical  necessity.  It  is  the  water  of  the  sweat  which  is  the 
most  important  excretion  from  the  skin,  although  the  elimi- 
nation of  CO2  and  inorganic  salts,  and  especially  of  urea  in 
some  pathological  conditions,  is  not  to  be  overlooked. 

14 


2IO  EXCRETION  BY  THE  KIDNEYS  AND  SKIN 

Structure. — The  skin  consists  of  an  external  covering,  the 
epidermis,  with  its  modifications,  hair  and  nails,  and  of  the 
cutis  vera.  Imbedded  in  the  cutis  vera  are  sebaceous  and 
sweat  glands  and  hair- follicles.  (Fig.  61.) 

Epidermis. — The  epidermis  consists  of  at  least  four  lay- 
ers of  epithelial  cells.  From  above  downward  these  are  ( I ) 
the  stratum  corneum}  (2)  the  stratum  lucidum,  (3)  the 
stratum  granulosum,  (4)  the  rete  mucosum  or  Malpighii. 
All  these  except  the  stratum  corneum  have  a  fairly  constant 
thickness.  The  stratum  corneum  is  thick  or  thin  according 
to  location  and  degree  of  exposure,  and  its  cells  are  flat  and 
horny.  The  lowest  cells  of  the  rete  mucosum  are  columnar. 
From  this  last-named  layer  the  cells  pass  gradually  upward, 
and  as  gradually  assume  the  shape  of  the  horny  layer.  The 
horny  cells  are  thrown  off  and  their  place  is  taken  by  others 
from  beneath.  (Fig.  60.) 

Hairs  are  to  be  found  on  almost  all  parts  of  the  cutaneous 
surface.  They  consist  of  a  bulb  and  a  shaft.  A  depression 
of  the  skin  involving  both  epidermis  and  cutis  vera  consti- 
tutes the  hair-follicle  in  which  the  bulb  rests.  A  projection 
at  the  bottom  of  the  follicle  corresponds  to  a  papilla,  and 
upon  it  the  bulb  is  placed.  The  shaft  has  an  oval  shape  in 
cross  section.  It  is  composed  of  fibrous  tissue,  outside 
which  is  a  layer  of  imbricated  cells. 

Nails  consist  of  a  superficial  layer  of  horny  cells  and  a 
deeper  one  corresponding  to  the  rete  mucosum.  The  root  of 
the  nail  is  received  into  the  matrix — a  specialized  portion 
of  the  cutis  vera. 

Cutis  Vera. — The  cutis  vera  is  tough  but  elastic.  It  rests 
upon  cellular  and  adipose  tissue.  Its  structure  is  areolar 
with  some  non-striated  muscle  fibers.  Projecting  from  the 
cutis  vera  into  the  epidermis  are  minute  conical  elevations, 
the  papilla.  Many  of  them  contain  sensory  nerve  terminals. 

Sweat  Glands. — Practically  the  whole  cutaneous  surface 
contains  sweat  glands.  Some  two  and  a  half  mil- 
lions are  thought  to  exist  in  the  skin  of  the  av- 


SWEAT  GLANDS 


211 


erage    individual.      They     are    particularly    abundant    in 
the     skin     of     the     palms     of     the     hands     and     soles 


FIG.  61. — Vertical  section  of  skin. 

A,  sebaceous  gland  opening  into  hair- follicle;  B,  muscular  fibers;  C,  sudorif- 
erous or  sweat-gland;  D,  subcutaneous  fat;  E,  fundus  of  hair- follicle,  with  hair- 
papilla.  (Kirkes  after  Klein.) 

of   the    feet.      They   belong  to   the    simple   tubular   type, 
and  consist  of  a  secreting  portion  and  an  excretory  duct. 


212  EXCRETION  BY  THE  KIDNEYS  AND  SKIN 

The  secreting  part  lies  just  underneath  the  true  skin  and,  as 
a  whole,  resembles  a  small  nodule ;  however,  the  nodule  con- 
sists of  an  intricate  coiling  of  the  tube  itself  which  is  of  ap- 
proximately uniform  diameter  throughout.  It  curls  upon 
itself  some  6-12  times  and  ends  by  a  blind  extremity.  It  is 
lined  by  epithelial  cells. 

The  duct  passes  away  from  the  glandular  coil,  runs 
through  the  cutis  vera.  in  a  comparatively  straight  course  and 
assumes  a  spiral  shape  as  it  traverses  the  epidermis  to  open 
obliquely  on  the  surface.  With  the  ducts  of  the  larger 
glands  are  connected  a  few  non-striped  muscular  fibers 
which  may  aid  in  the  discharge  of  the  secretion.  (Fig.  61.) 

Properties  and  Composition  of  Sweat. — The  secretion  is 
colorless,  has  a  slight  characteristic  odor,  and  a  salty  taste. 
Its  specific  gravity  is  about  1003-4,  and  its  reaction  is  usu- 
ally acid  when  just  discharged.  It  contains  a  large  propor- 
tion of  water,  a  little  urea  and  fatty  matter,  and  quite  a 
quantity  of  inorganic  salts  of  which  the  chief  is  sodium 
chloride.  All  the  constituents  in  health  are  of  subsidiary  im- 
portance except  the  water.  Under  average  conditions  of 
temperature  and  exercise  the  amount  secreted  in  24  hours 
is  about  2  pounds.  But  the  quantity  is  very  variable — as 
much  so  as~the  urine,  and  may  be  said  in  a  general  way  to 
vary  inversely  as  the  urinary  secretion. 

Mechanism  of  the  Secretion  of  Sweat. — Sweat  is  pro- 
duced continuously,  though  up  to  a  certain  point  it  passes 
off  as  vapor  or  "insensible  perspiration."  Beyond  that  point 
it  accumulates  on  the  skin  as  an  evident  fluid  and  becomes 
"sensible  perspiration."  Whether  it  escapes  as  sensible  or 
insensible  perspiration,  it  is  secreted  as  a  fluid. 

The  activity  of  the  cells  lining  the  glandular  coils  in  sep- 
arating sweat  from  the  blood  is  undoubted.  Distinct  secre- 
tory fibers  are  distributed  to  them,  and  through  the  influence 
of  these  fibers  the  glands  will  secrete  sweat  even  without  an 
increase  in  the  blood  supply.  But  usually  a  determination 
of  blood  to  the  surface  means  an  increase  of  perspiration. 


SECRETION  OF  SWEAT  213 

This  occurs  during  violent  exercise,  e.  g.  However,  that 
the  production  of  sweat  is  not  altogether  dependent  on  this 
factor  is  shown  by  profound  sweating  in  shock,  nausea  and 
like  conditions  when  the  skin  is  pale  and  cold,  and  by  dry- 
ness  of  the  flushed  skin  in  febrile  diseases.  Furthermore, 
experiments  on  inferior  animals  have  revealed  fibers  which 
influence  the  secretion  of  sweat  without  affecting  the  blood 
flow. 

Practically,  in  health,  the  only  conditions  which  increase 
the  flow  of  perspiration  are  muscular  exercise  and  a  high  ex- 
ternal temperature.  Of  these,  exercise  probably  works 
through  the  nerve  centers;  external  heat  does  not  stimulate 
the  glands  directly,  but  irritates  the  cutaneous  terminations 
of  afferent  fibers  which  convey  impressions  to  the  sweat  cen- 
ters, whence  messages  are  sent  out  by  secretory  (fibers  to  the 
glandular  epithelium  and  their  activity  begins.  In  both  cases 
ihere  is  accompanying  dilatation  of  the  superficial  vessels 
under  the  influence  of  the  vaso-dilator  fibers. 

It  is  supposed  that  the  chief  center  is  in  the  medulla  ob- 
longata  and  that  secondary  centers  exist  in  the  lumbar  re- 
gion of  the  cord. 

The  amount  of  CO2  eliminated  by  the  skin  is  inconsider- 
able in  the  human  being. 


CHAPTER  XL 
THE  NERVOUS  SYSTEM. 

General  Functions  of  the  System  as  a  Whole. — The  ner- 
vous system  is  the  most  delicately  organized  part  of  the  ani- 
mal body.  Its  sensory  terminations  receive  impressions 
which  are  conducted  to  the  centers;  it  conveys  impulses 
from  the  centers  to  the  different  parts  of  the  body,  control- 
ling and  regulating  their  action.  Connecting,  as  it  does,  all 
parts  of  the  organism  into  a  coordinate  whole,  it  is  the  only 
medium  through  which  impressions  are  received,  and  is  the 
only  agency  through  which  are  regulated  movement,  secre- 
tion, calorification  and  all  the  processes  of  organic  life.  This 
system,  ramified  throughout  the  body,  connected  with  and 
passing  between  its  various  organs,  serves  them  as  a  bond 
of  union  with  each  other,  as  well  as  with  the  brain.  The 
mind  influences  the  corporeal  organs  through  the  instru- 
mentality of  this  system,  as  when  volition  calls  them  into  ac- 
tion; on  the  other  hand,  changes  in  the  organs  of  the  body 
may  affect  the  mind  through  the  same  channel,  as  when,  for 
instance,  pain  is  mentally  perceived  when  the  finger  is 
burned.  Thus  it  is  that  the  nervous  system  becomes  the 
main  agent  in  what  is  known  as  the  "life  of  relation" ;  for 
without  some  medium  for  the  transmission  of  its  mandates, 
or  some  means  of  receiving  those  impressions  which  exter- 
nal objects  are  capable  of  exciting,  the  mind  would  be  com- 
pletely isolated,  and  could  hold  no  communion  with  the  ex- 
ternal world. 

It  should  not  be  understood,  however,  that  the  nervous 
system  cannot  operate  independently  of  mental  influence.  All 
those  manifestations  of  nervous  activity  connected  with  the 

214 


GENERAL  FUNCTIONS  215 

performance  of  the  so-called  "organic  functions"  of  life  as 
digestion,  circulation,  etc.,  are  not  directly  influenced  by  vo- 
lition; indeed  an  essential  character  of  these  functions  is 
that  they  are  completely  removed  from  the  influence  of  the 
will ;  to  be  conscious  subjectively  of  their  performance  is  an 
evidence  of  abnormality. 

The  first  step  in  every  voluntary  act  is  a  mental  change, 
in  which  the  act  of  volition  consists.  If  this  mental  change 
be  of  such  nature  as  to  direct  its  influence  upon  a  muscle, 
or  a  particular  set  of  muscles,  the  contraction  of  those  mus- 
cles immediately  supervenes,  so  as  to  bring  about  the  prede- 
termined voluntary  act.  But  the  influence  of  the  will 
could  not  possibly  be  exerted  upon  those  muscles  except 
through  intervention  of  the  nerves. 

Furthermore,  a  certain  mental  state,  in  cases  of  common 
or  special  sensation,  is  induced  by  an  impression  made  upon 
certain  bodily  organs.  But  in  no  case  could  the  mental  state 
be  produced  unless  a  particular  part  of  the  nervous  system 
were  present  to  convey  the  impression  received  to  the  center 
capable  of  recognizing  it.  If  the  hand  be  burned  pain  is  felt, 
but  were  the  nerves  not  present  to  convey  the  impression 
made  by  the  heat  no  degree  of  temperature  could  make  the 
mind  cognizant  of  injury.  When  light  is  admitted  to  the 
eye  a  corresponding  mental  sensation  is  produced,  but  for 
the  production  of  this  the  integrity  of  the  optic  nerve  is  a 
necessary  condition. 

It  will  be  gathered  from  the  foregoing  remarks  that  the 
nervous  system  is  not  only  capable  of  conveying  communi- 
cations, but  that  it  has  the  power,  in  certain  of  its  divisions, 
of  receiving  impressions  and  of  giving  rise  to  stimulating  in- 
fluences— that  is,  that  it  is  capable  of  generating  a  peculiar 
power  known  as  "nerve  force."  It  thus  becomes  the  seat  of 
distribution  of  energy  to  all  the  cells.  These  generating 
parts  of  the  system  are  the  reservoirs  of  force — force  which 
has  been  derived  from  the  cells  and  is  distributed  to  them. 
This  nervous  force,  having  its  origin  in  the  living  activity 


2l6  THE  NERVOUS  SYSTEM 

of  the  cells,  is  the  highest  manifestation  of  vital  energy. 
The  nervous  structure  is  divided  into  two  great  systems: 

1.  The  Cerebro-spinal  System  consists  of  the  brain,  the 
spinal  cord  and  all  the  nerves  which  run  off  from  these.  This 
system  is  especially  concerned  with  the  functions  of  relation, 
or  of  animal  life.    It  presides  over  general  and  special  sen- 
sation, over  voluntary  movements,  over  intellection,  over  all 
conscious  activity,  and  over  all  other  functions  which  are 
peculiar  to  the  animal.     It  is  by  this  system  that  we  know 
of  and  deal  with  the  other  great  system. 

2.  The  Sympathetic,  or  Ganglionic  System  is  especially 
connected  with  the   functions   relating  to  nutrition — func- 
tions similar  to  those  occurring  in  the  vegetable  kingdom. 
It  presides  over  all  organic  life — over  all  unconscious  ac- 
tivity.     While  the  operations  over  which  this  system  holds 
sway  are  quite  different  from  those  under  the  supervision 
of  the  cerebro-spinal  system,  it  must  not  be  concluded  that 
the  two  are  not  anatomically   and  physiologically   related. 
Neither  is  independent  of  the  other,  as  was  once  thought,  but 
both  are  parts  of  the  same  great  apparatus. 

Divisions  of  the  Nervous  Substance  as  a  Whole. — The 
nervous  matter,  irrespective  of  the  two  systems,  may  be 
studied  as  consisting  of  two  divisions.  The  first  is  made  up 
of  cells;  the  second  of  tubes,  or  fibers.  Although  the  tissue 
may  be  thus  divided  into  nerve  cells  and  nerve  fibers,  the 
present  conception  of  the  arrangement  of  the  nervous 
substance  is  that  these  two  are  only  different  parts  of  the 
same  element  known  as  the  neuron,  supported  by  tissue  ele- 
ments known  as  neuroglia,  which,  though  not  identical  with 
connective  tissue,  is  comparable  to  it  in  its  function  of  sup- 
port. The  neuron,  thus  considered,  consists  of  a  proto- 
plasmic body  which  sends  out  a  number  of  branching  pro- 
cesses called  dendrites,  one  of  which  becomes  the  axis  cylin- 
der. While,  therefore,  it  is  to  be  understood  that  the  cell 
and  the  fiber  in  the  nervous  system  are  both  portions  of  an 
identical  whole,  a  description  of  them  as  separate  parts  is 


NERVE  FIBERS  217 

warranted  for  the  sake  of  convenience  and  by  differences  in 
their  general  characteristics. 

The  nerve  cells  are  the  only  organs  capable,  under  any  cir- 
cumstances, of  generating  nerve  force.  As  a  rule  they  are 
stimulated  to  generate  this  force  by  the  reception  of  an  im- 
pression through  the  nerve  fiber,  but  they  may  in  some  cases 
be  directly  excited  by  mechanical,  electrical  or  chemical 
means.  They  also  frequently  act  as  conductors,  as  will  be 
seen  later. 

Under  no  circumstances  can  nerve  fibers  generate  force. 
Their  office  is  exclusively  to  conduct  impressions  and  im- 
pulses, and  they  usually  receive  these  impressions  and  im- 
pulses at  their  terminal  extremities*  in  the  case  of  afferent 
nerves,  and  from  the  centers  in  the  case  of  efferent  nerves ; 
but  in  many  instances  they  may  be  stimulated  in  any  part  of 
their  course.  Some  fibers  are  incapable  of  being  thus  di- 
rectly stimulated.  The  nerves  of  special  sense  are  insensi- 
ble to  direct  stimulation. 

Nerve  Fibers. — Nerve  fibers  are  of  two  kinds:  (A)  white 
or  medullated  fibers  and  (B)  gray  or  non-medullatcd  fibers. 
The  non-medullated  fibers  possess  the  conducting  elements 
alone,  while  the  medullated  possess  certain  accessory  ana- 
tomical elements. 

(A)  Each  medullated  fiber  has  (i)  an  external  envelop- 
ing membrane  called  the  ncurilemma,  or  the  primitive  nerve 
sheath,  or  the  sheath  of  Schwann;  (2)  an  intermediate  sub- 
stance known  as  the  myeline  sheath,  or  the  white  substance 
of  Schwann,  or  the  medullary  substance;  (3)  a  central 
fiber,  the  true  conducting  element,  which  usually  goes  under 
the  name  of  the  axis  cylinder,  or  axone. 

The  sheath  of  Schwann  is  analogous  to  the  sarcolemma 
of  muscle  fibers.  It  is  a  structureless  protective  membrane, 
somewhat  elastic,  and  presents  oval  nuclei  with  their  long 
diameter  corresponding  to  the  direction  of  the  fiber.  This 
sheath  is  wanting  over  the  medullated  fibers  in  the  white  sub- 
stance of  the  brain  and  spinal  cord, 


218 


THE  NERVOUS  SYSTEM 


Node  of  Ranvier. 


Primitive  sheath. 


Nerve  corpuscles. 


Axis  cylinder. 


White  substance 
of  Schwann. 


Node  of  Ranvier. 


FIG.  62. — Scheme  of  a 
medullated  nerve  fiber  of 
a  rabbit  acted  on  by 
osmic  acid. 

The  incisures   are   omitted. 
X  400.      (Landois.) 


It  is  the  white  substance  of 
Schwann  which  gives  to  the  nerve 
its  peculiar  whitish  appearance. 
This  is  a  fatty  substance  of  a  semi- 
fluid consistence.  It  fills  the  tube 
made  by  the  sheath  of  Schwann 
and  surrounds  the  axis  cylinder. 
It  is  wanting  at  the  origin  of  the 
fibers  in  the  centers  and  at  their 
peripheral  distribution.  It  is  prob- 
ably not  necessary  to  conductivity. 
In  fresh  nerves  this  substance  is 
'strongly  refractive,  and  the  optical 
effect  produced  by  its  varying 
thickness  in  the  center  and  at  the 
edges  is  the  appearance  of  dark 
borders.  It  easily  coagulates  into 
an  opaque  mass.  The  idea  that  the 
myeline  sheath  acts  as  an  insulator 
lacks  supporting  evidence.  The 
theory  that  it  is  nutritional  is 
plausible;  but  no  sufficient  differ- 
ence in  the  medullated  and  non- 
medullated  fibers  in  this  respect 
has  been  found  to  establish  the  the- 
ory as  a  fact.  At  certain  points  in 
in  the  course  of  medullated  fibers 
there  are  seen  constrictions  called 
the  nodes  of  Ranvier.  At  these 
points  the  medullary  substance  is 
wanting  and  the  sheath  of 
Schwann  is  in  contact  with  the 
axis  cylinder.  It  is  not  improbable 
that  these  nodes  furnish  a  mode  of 
access  for  the  nutrient  plasma. 
Certain  it  is  that  they  are  most 


NERVE  TRUNKS 


2IQ 


numerous  where  the  physiological  activity  is  supposed  to  be 
most  active. 

The  axis  cylinder  is  composed  of 
a  large  number  of  primitive  fibrillae. 
This  band  occupies  about  one-fourth 
the  diameter  of  the  tube  and  is  the 
true  conducting  element,  as  is  shown 
by  its  invariable  presence,  its  contin- 
uity and  other  considerations  equally 
conclusive.  It  is  demonstrated  under 
the  microscope  with  difficulty  in  fresh 
specimens.  It  is  directly  connected 
with  a  nerve  cell,  and  is  the  essential 
part  of  the  fiber.  The  process  of  the 
cell  which  becomes  the  axis  cylinder 
is  not,  as  was  once  thought  unbranch- 
ed,  but  itself  sends  off  "collaterals" 
in  the  gray  substance.  These  collat- 
erals, however,  do  not  actually  join 
any  other  nerve  cells  or  fiber. 

The  average  diameter  of  medullated 
fiber  is  about  ^ooo  in.,  though  all  are 
said  not  to  preserve  the  same  diam- 
eter throughout  their  course. 

(B)  The  non-medullated  fibers 
(fibers  of  Remak)  seem  to  be  simple 
axis  cylinders  without  the  other  atom- 
ical  elements  peculiar  to  medullated 
fibers.  They  make  up  a  large  part  of 
the  trunks  and  branches  of  the  sym- 
pathetic system,  and  represent  the  fil- 
aments of  origin  and  distribution  of  FIG.  63.— Non-medullated 
all  nerves.  They  are  thought  by  some  nerve  fiber- 

to  possess  a  neurilemma.     They  are    u>  VnucfeuSf;  d^ 
pale  gray  in  color.  surrounding  it. 

Nerve   Trunks. — The  above   remarks   apply  to  a   single 


22O 


THE  NERVOUS  SYSTEM 


nerve  fiber.  These  fibers  seldom  run  an  extended  course 
alone,  but  are  bound  together  in  large  numbers  to  make  a 
nerve  trunk.  This  trunk  is  composed  of  a  number  of 
bundles  of  fibers,  and  is  surrounded  by  a  connective  tissue 
membrane  known  as  the  epineurium;  the  separate  bundles, 
or  funiculi,  are  surrounded  each  by  a  similar  membrane 
called  the  perineurium;  while  inside  the  funiculi,  between 


FIG.  64. — Transverse  section  of  a  nerve.     (Median.) 

ep,  epineurium;  pe,  perineurium;  ed,  endoneurium.     (Landois.) 

the  primitive  fasciculi,  is  a  delicate  supporting  tissue  known 
as  the  end  on  curium,  or  the  sheath  of  Henle.  In  connection 
with  this  sheath  there  are  nuclei  belonging  to  the  connective 
tissue  and  to  the  nerve  fibers  themselves.  The  sheath  be- 
gins where  the  nerve  fibers  emerge  from  the  white  portion 
of  the  centers,  is  interrupted  by  the  ganglia  in  the  course  of 
the  fibers,  branches  as  the  bundle  branches,  and  is  lost  before 
the  terminal  distribution  is  reached.  It  is  seldom  found  sur- 
rounding single  fibers.  It  is  likewise  rare  for  capillaries  to 


NERVE  CELLS  221 

penetrate  it  and  reach  the  fibers  themselves.  There  are  nu- 
merous lymph  spaces  around  the  individual  fibers  as  well 
as  around  the  funiculi.  In  situations  where  the  nerves  are 
well  protected,  as  in  the  cranium,  the  amount  of  fibrous 
tissue  in  the  trunks  is  small,  but  where  opposite  conditions 
prevail,  as  in  muscular  substance,  this  tissue  is  largely  in- 
creased in  amount  as  regards  both  that  which  surrounds  the 
trunk  and  that  which  is  sent"  in  between  the  funiculi  and 
fiber.  This  tissue  has  ramifying  in  it  a  network  of  fibers 
known  as  neri  nervorum.  The  blood  supply  is  not  large. 

Individuality  of  Nerve  Fibers. — It  is  to  be  remembered 
that  so  far  as  can  be  determined  every  nerve  fiber,  having 
entered  a  trunk,  proceeds  without  interruption  to  the  part  to 
which  it  is  finally  distributed,  whether  that  part  be  the  skin, 
or  a  viscus,  or  a  muscle,  or  a  gland,  or  some  organ  of  special 
sense,  or  another  nerve  cell,  or  what  not.  Collections  of 
fibers  forming  bundles  run  together  in  the  same  trunk,  may 
leave  that  trunk  together,  may  send  out  part  of  their  fibers  to 
another  bundle  or  trunk,  or  may  receive  other  fibers  from 
other  funiculi ;  but  everywhere  the  relation  of  the  primitive 
fibers  to  each  other  is  simply  one  of  contiguity.  Hbwever, 
as  the  axis  cylinder  approaches  the  seat  of  its  final  distribu- 
tion, it  breaks  up  into  several  fibrillse,  such  divisions  always 
taking  place  at  the  nodes  of  Ranvier. 

Nerve  Centers. — The  nerve  centers  include  the  gray  mat- 
ter of  the  brain  and  cord  and  the  ganglia  in  both  the  cerebro- 
spinal  and  sympathetic  systems.  These  centers  have  a  gray 
color  due  to  the  presence  of  a  pigmentary  substance  in  the 
cells  and  surrounding  tissue.  The  ganglionic  centers  are 
simple  collections  of  nerve  cells  with  their  usual  accessory 
elements — myelocytes,  intercellular  granular  matter,  delicate 
membranes  covering  some  of  the  cells,  connective  tissue  ele- 
ments, blood-vessels  and  lymphatics. 

Nerve  Cells. — These  are  irregular  in  shape  and  may  be 
unipolar,  bipolar  or  multipolar.  They  also  vary  much  in 
size.  The  unipolar  cell  has  a  single  prolongation  which  be- 


222 


THE  NERVOUS  SYSTEM 


comes  the  axis  cylinder.  Bipolar  cells  are  prolonged  in  two 
directions,  and  may  be  looked  upon  as  simply  protoplasmic 
enlargements  of  the  nerve  fiber.  This  cell  is  frequently  cov- 
ered by  a  connective  tissue  envelope  which  is  continuous  in 
both  directions  with  the  sheath  of  Schwann.  Multipolar 


Dendrites. 


Nerve-cell.  « 


Nerve  process, 
or  axone. 


Neurilemma.  — 


Neurilemma. 


•  Neive-cell. 


FIG.  65. 

A,  efferent  neuron;  B,  afferent  neuron.     (Brubaker.) 

cells  have  three  or  more  prolongations,  one  of  which  always 
becomes  continuous  with  the  axis  cylinder  and  is  called  the 
axis-cylinder  process,  the  neuraxon,  or  the  axone.  The 
other  poles  branch  in  various  irregular  directions  like  the 
limbs  of  a  tree,  and  are  hence  called  dendrites.  They  also 
go  under  the  name  of  protoplasmic  prolongations.  Some  of 
these  unite  the  cells  to  contiguous  cells  by  interlacing  with, 


NEURONS  223 

but  not  actually  joining,  similar  poles  from  those  cells.  The 
multipolar  cells  in  the  anterior  cornua  of  gray  matter  of  the 
cord  are  said  to  be  larger  in  size  and  to  present  more  poles 
than  corresponding  cells  in  the  posterior  column. 

The  diameter  of  nerve  cells  varies  from  M.250  to  %oo  in. 
The  nucleus  is  usually  single,  and  most  cells  have  no  true 
surrounding  membrane.  If  a  nerve  fiber  be  followed  toward 
the  center  which  gives  it  origin  it  will  be  found  first  to  lose 
its  sheath  and  later  its  medullary  substance ;  this  medullary 
substance  may  continue  for  some  distance  after  the  sheath 
is  lost,  as  in  the  white  substance  of  the  encephalon,  but  never 
penetrates  the  gray  substance  proper.  Every  nerve  fiber 
is  connected  with  a  cell  by  that  cell's  axis-cylinder  prolon- 
gation. 

Certain  retrograde  changes  take  place  in  the  neurons  in 
old  age — morphological  changes  agreeing  with  the  physio- 
logical decrease  in  energy-producing-  power  at  that  time. 
The  cell  body  becomes  smaller,  the  dendrites  atrophy,  and 
the  axones  diminish  in  mass.  Nerve  "fatigue"  can  also  be 
demonstrated  by  the  microscope.  The  nuclei  of  the  sheath 
are  flattened,  the  protoplasm  is  shrunken  and  vacuolated 
and  the  nucleus  is  crenated.  The  quantity  and  quality  of 
the  food  may  be  perfect,  but  the  power  of  the  cell  to  utilize 
it  is  impaired,  and  this  means  diminished  physiological 
power. 

Communication  Between  Different  Neurons. — Every  neu- 
ron is  anatomically  independent  of  every  other  neuron. 
There  is  no  actual  joining  of  fibers  or  dendrites — simply  an 
interlacement  of  the  end  arborizations.  This  is  illustrated 
in  Figs.  62  and  63.  In  the  latter  the  afferent  fiber  is  joined 
to  no  cell  except  G,  one  of  the  cells  of  the  spinal  root  gang- 
lion. Its  end  arborizations  simply  interlace  with  the  dend- 
rites of  the  motor  cell  M.  C.  and  cause  it  to  send  out  an 
efferent  impulse  to  the  muscle  M. 

Furthermore,  there  are  frequent  relays  in  the  transmission 
of  nerve  messages.  By  no  means  do  all  the  fibers  from  the 


224 


THE  NERVOUS  SYSTEM 


B.C. 


M,C: 


FIG.  66. — Reflex  action;  old  idea.     (Kirkcs.) 


FIG.  67. — Reflex  action;  modern  idea.     (Kirkes.) 


NERVE  FIBERS 


motor  area  of  the  brain  pass  them- 
selves out  as  parts  of  the  anterior 
roots.  The  relay  service  is  illus- 
trated in  Fig.  64.  Here  again,  it  is 
seen  that  there  is  no  actual  joining  of 
the  neurons.  Whenever  it  is  said  that 
a  nerve  cell  is  "joined"  to  another,  or 
that  the  axis  cylinder  of  a  cell  "joins" 
another  cell,  no  actual  continuity  of 
tissue  is  meant.  Different  neurons 
communicate  only  by  contiguity. 

Peripheral  Nerve  Terminations. — 
Nerves  terminate  peripherally  (r)  in 
muscles,  (2)  in  glands,  (3)  in  special 
organs  connected  with  the  senses  of 
sight,  hearing,  smell  and  taste,  (4) 
in  hair-follicles,  (5)  in  simple  free 
extremities  passing  between  epithelial 
and  other  cells,  and  (6)  in  several 
kinds  of  so-called  tactile  corpuscles. 

The  motor  nerves  passing  to  vol- 
untary muscles  form  first  a  "ground 
plexus"  for  each  group  of  muscle 
bundles — this  plexus  being  made  of 
the  axis-cylinder  fibrillae.  From  this 
plexus  fibrils  pass  to  form  an  "inter- 
mediary plexus"  corresponding  to 
each  muscle  bundle.  These  fibrils  are 
still  medullated,  and  when  a  branch 
from  the  intermediary  plexus  enters  a 
muscle  fiber  its  sheath  becomes  con- 
tinuous with  the  sarcolemma  of  that 
fiber,  and  the  axis-cylinder  fibrils 
form  a  network  on  the  surface  of 
the  muscle  fiber.  This  is  called  an 
end  motorial  plate.  It  contains  a 


15 


s.c; 


[M 

FIG.  68. — Diagram  of  an 
element  of  the  motor  path 

U.  S.,  upper  segment;  L. 
S.,  lower  segment;  C.C., 
cell  of  cerebral  cortex;  S.C., 
cell  of  spinal  cord,  in  ante- 
rior cornu;  M,  the  muscle; 
S,  path  from  sensory  nerve 
roots.  (Kirkes  after  Cowers.) 


226  THE  NERVOUS  SYSTEM 

number  of  nuclei,  and  sends  off  from  its  under  surface  fine 
fibrillae  which  are  said  to  pass  between  the  muscular  fibrillse 
which  make  up  the  fiber.  Sensory  fibers  are  somewhat 
scantily  distributed  to  the  voluntary  muscles. 

In  plain  muscle  tissue  the  motor  nerves  are  distributed 
after  the  same  general  manner  as  in  the  striped  muscles, 


Nerve-fibre. 


End-plate. 


Muscle  nucleus. 


FIG.  69. — Termination  of  a  nerve  fiber  in  end-plate  of  a  lizard's 
muscle.     (Stirling.) 

though  with  some  differences.  Here  the  fibers  are  not  me- 
dullated,  and  primitive  fibrils  passing  from  the  intermediary 
plexus  finally  enter  the  nuclei  of  the  muscle  cells. 

Medullated  fibers  have  been  traced  to  the  cells  of  glands, 
but  not  farther.  It  is  thought  by  some  that,  having  formed 
a  plexus,  non-medullated  fibers  pass  in  to  terminate  in  the 
nucleoli  of  the  gland  cells,  though  such  endings  have  not 
been  demonstrated. 

The  peripheral  distribution  of  nerves  connected  with  the 
special  senses  will  be  discussed  elsewhere. 

The  remaining  methods  of  termination  above  noted  apply 
to  afferent  nerves.  It  is  claimed  that  a  very  large  number  of 
sensory  nerves  terminate  in  hair-follicles.  If  such  be  the 
case  it  will  account  for  sensory  terminations  in  by  far  the 
greater  part  of  the  cutaneous  surface.  It  is  supposed  that 


NERVE  FIBERS 


227 


nerve  fibrillse  form  a  plexus  beneath  the  true  skin  and  send 
branches  thence  to  the  follicles,  though  the  exact  mode  of 
termination  is  a  question  of 
some  obscurity. 

Terminations  between  epi- 
thelial cells  are  probably  more 
common  than  any  other  meth- 
od of  sensory  distribution. 
The  fibers,  having  passed  to 
the  surface  of  the  skin  or  mu- 
cous membrane,  lose  every- 
thing excepting  the  axis  cylin- 
der, which,  dividing  into  mi- 
nute ramifications,  passes,  by 
means  of  these  fibrillae, 
among  the  epithelial  cells. 
This  mode  of  termination  is 
held  by  some  to  prevail  in  the 
glands.  It  certainly  prevails 
in  parts  other  than  the  skin 
and  mucous  membranes. 

Sensory  nerves  further  ter- 
minate in  (a)  the  corpuscles 
of  P acini  or  Vater,  (b)  the  end 
bulbs,  or  tactile  corpuscles  of 
Krause,  (c)  the  tactile  corpus- 
cles of  Meissner,  (d)  the 
tactile  menisques,  and  (e)  the 
corpuscles  of  Golgi. 

(a)  The  Pacinian  Corpus- 
cles are  oval  elongated  bodies- 
Each  corpuscular  body  has  a 


FIG.  70. — Vater's  or  Pacini's 
corpuscle. 


a,  stalk;  b.  nerve  fiber  entering  it; 

-         P"   ,         ,    -,,         r~        •       i          c,    d.,    connective-tissue   envelope;    e, 
length  of  about  1/12  of  an  inch,      axis-cylinder    with    its    end    divided 

and  is  about  half  as  broad.   It 


at  /.      (Landois.) 


is  made  up  of  a  number  of  concentric  layers  of  connective 
tissue  in  a  hyaline  ground  substance  and  is  attached  by  a 


228 


THE  NERVOUS  SYSTEM 


pedicle  to  the  nerve  whose  termination  it  is.  Through  this 
pedicle  passes  a  single  (occasionally  more)  nerve  fiber 
which,  piercing  the  several  concentric  layers  constituting 
the  corpuscle,  gradually  loses  its  myeline  substance  and  runs 
longitudinally  through  the  center  of  the  body  to  terminate  at 
the  distal  end  of  the  central  cavity  in  a  knob-like  enlarge- 
ment. These  corpuscles  are  found  in  great  abundance  on  the 
palmar  and  plantar  surfaces  of  the  hands  and  feet,  being  far 
more  numerous  on  the  first  phalanx  of  the  index  finger  than 
elsewhere.  About  six  hundred  are  said  to  be  present  in  each 
hand  and  foot.  They  are  also  to  be  found  on  the  dorsal  sur- 
faces of  the  hands  and  feet,  over  parts  of  the  forearm,  arm 
and  neck,  in  the  nipples,  in  the  substance  of  muscles,  in  all 

the  great  plexuses  of  the  sympa- 
thetic system,  and  in  numerous 
other  situations.  These  bodies  can- 
not be  considered  true  tactile  cor- 
puscles because  they  are  situated 
beneath  the  skin ;  neither  can  they 
be  positively  said  to  have  any  "spe- 
cial sensory"  function  such  as  the 
appreciation  of  temperature, 
weight,  etc. 

(b)  The  end  bulbs  of  Krause 
exist  in  great  number  in  the  con- 
junctiva, the  glans  penis  and  cli- 
toris, the  lips,  and  in  other  situa- 
tions. They  bear  some  resem- 
blance to  the  corpuscles  of  Pacini, 
but  are  much  less  elaborate  in  their  arrangement;  the  num- 
ber of  concentric  layers  is  much  smaller,  while  the  contained 
mass  is  larger.  The  shape  is  spherical.  From  one  to  three 
medullated  fibers  pass  from  the  underlying  plexus  to  wind 
through  the  corpuscle  and  break  up  in  free  extremities.  The 
sheath  of  the  fiber  is  continuous  with  the  outer  covering  of 
the  corpuscle,  and  the  medulla  is  gradually  lost  as  the  fiber 


FIG.  71. — End  bulb  from 
human  conjunctiva,  treat- 
ed with  osmic  acid,  show- 
ing cells  of  core.  (From 
Yeo  after  Longivorth.) 

a,  nerve  fiber;  b,  nucleus  of 
sheath;  c,  nerve  fiber  within 
core;  d,  cells  of  core. 


NERVE  FIBERS 


229 


enters  the  bulb.     The  end  bulb  of  Krause  measures  from 
Hooo  to  ^so  of  an  inch  in  diameter. 

(c)  The  tactile  corpuscles  of  Meissner  have  to  do  with  the 
sense  of  touch,  and  are  situated  largely  in  the  papillae  of  the 
skin  covering  the  palmar  surfaces  of  the  hands  and  the 
plantar  surfaces  of  the  feet;  they  also  exist  in  other  situa- 
tions, corresponding  in  general  to  the  distribution  of  the 
Pacinian  corpuscles.  The  largest  number  is  found  over  the 
distal  phalanges  of  the  fingers  and  toes  on  their  palmar  and 


FIG.  72. — Drawing  from  a  section  of  injected  skin. 

Showing  three  papillae,  the  central  one  containing  a  tactile  corpuscle,  a,  which 
is  connected  with  a  medullated  nerve,  and  those  at  each  side  are  occupied  by 
vessels.  (From  Yeo  after  Cadiat.) 

plantar  surfaces ;  they  diminish  in  number  proximally  from 
these  points.  They  may  be  simple  or  compound  according 
as  the  enclosing  capsule  contains  one  or  more  collections  of 
nucleated  cells.  Their  form  is  oblong  with  the  long  axis 
in  the  direction  of  the  papillae.  They  vary  in  thickness  with 
the  papillae  of  the  region  in  which  they  are  located.  They 
may  have  a  transverse  diameter  of  from  %oo  to  M.50  of  an 
inch,  and  probably  in  most  instances  occupy  the  secondary 
eminences  of  the  papillae  in  which  they  are  found.  A  simple 
papilla  does  not  generally  possess  both  vascular  and  nervous 
loops. 

(d)  The  tactile  menisques  are  found  in  certain  cutaneous 
regions.    Nerves  in  the  superficial  layer  of  the  skin  lose  their 


230  THE  NERVOUS  SYSTEM 

medullary  substance  and  divide  to  form  arborization  which 
are  flattened  into  the  form  of  a  leaf. 

(e)  The  corpuscles  of  Golgi  are  situated  at  the  point  of 
union  of  tendons  with  muscles,  and  are  believed  by  some  to 
have  to  do  with  the  muscular  sense.  They  are  flattened  fusi- 
form bodies  composed  of  granular  substance  enclosed  in 
layers  of  hyaline  membrane  and  containing  nervous  fibrillae. 

Properties  and  Classification  of  Nerve  Fibers. — Nerve 
fibers  are  for  the  purpose  of  conveying  messages  either  peri- 
pherally or  centrally.  They  may  be  stimulated  to  action  by 
anything  capable  of  suddenly  increasing  their  irritability.  In 
any  case  the  effect  of  the  stimulus,  whether  normal  or  ab- 
normal, is  manifested  at  the  peripheral  distribution  of  the 
stimulated  fiber.  So  far  as  most  external  manifestations  are 
concerned,  nerves  may  be  classified  as  motor  and  sensory. 
That  is  to  say,  stimulation,  for  instance,  of  a  cerebro-spinal 
nerve  (except  those  of  special  sense)  is  followed,  under  or- 
dinary conditions,  by  one  of  two  results — there  is  either 
pain  or  contraction  of  a  muscle  to  which  the  nerve  is  dis- 
tributed. This  is  a  typical  illustration  of  the  action  of  motor 
and  sensory  fibers,  and  the  manifestation  of  nerve  action, 
whether  it  consists  in  pain  or  motion,  is  a  result  only  of  the 
conduction  of  an  impression  of  an  impulse  to  the  center  or 
the  periphery.  It  is  to  be  noted  that  the  result  of  thus  stim- 
ulating a  nerve  fiber  is  manifested  at  one  extremity  only  of 
that  fiber,  and  always  at  the  same  extremity. 

However,  since  there  are  nerve  fibers  the  stimulation  of 
which  is  not  followed  by  pain  or  motion,  the  division  into 
sensory  and  motor  fibers  is  not  comprehensive  enough  to  in- 
clude all  the  fibers  in  the  body.  But  since,  as  above  stated, 
the  only  office  of  fibers  is  to  conduct,  and  since  they  always 
conduct  in  a  direction  either  tozvard  or  away  from  the  cen- 
ter, all  nerves  may  be  classified  as  either  centripetal  or  cen- 
trifugal. A  corresponding  division  is  into  afferent  and 
efferent.  It  will  be  seen  that  all  motor  fibers  are  centrifu- 
gal or  efferent,  but  not  all  centrifugal  or  efferent  fibers  are 


EFFERENT   NERVES  23! 

motor.  It  will  likewise  be  seen  that  all  sensory  fibers  are 
centripetal  or  afferent,  but  not  all  centripetal  or  afferent 
fibers  are  sensory.  For  impressions  made  upon  the  termina- 
tions, or  upon  the  trunk,  of  a  centripetal  nerve  may  cause 
(i)  pain,  or  some  other  kind  of  sensation;  (2)  special  sen- 
sation; (3)  renex  action  of  any  kind;  (4)  inhibition.  Simi- 
larly impressions  made  upon  a  centrifugal  nerve  may  (i) 
cause  contraction  of  a  muscle  (motor  nerve)  ;  (2)  influence 
nutrition  (trophic  nerve)  ;  (3)  control  secretion  (secretory 
nerve)  ;  (4)  inhibit,  augment,  or  stop  any  other  efferent  ac- 
tion (Kirkes). 

To  these  two  classes,  efferent  and  afferent,  should  be 
added  a  third,  the  intercentral  fibers  which  connect  different 
parts  of  the  nervous  centers.  Most  of  these  even  can  be 
called  either  afferent  or  efferent. 

Characteristics  of  Efferent  Nerves. — In  case  of  these 
nerves  a  force  is  generated  in  the  centers  and  conveyed  by 
the  nerves  to  the  periphery,  where  it  manifests  itself  in  one 
of  the  ways  mentioned  above  as  characteristic  of  centrifugal 
fibers.  Division  of  these  fibers,  or  interference  with  their 
conductivity  by  disease  or  otherwise,  renders  impossible  the 
manifestation  of  nervous  force  generated  in  the  center,  for 
the  simple  reason  that  the  organ  to  which  the  fibers  are  di- 
tributed  cannot  receive  the  message  intended  for  it.  For  in- 
stance, a  muscle  cannot,  by  the  most  persistent  effort  of  the 
will  be  made  to  contract  if  the  motor  fibers  running  to  that 
muscle  are  divided.  In  case,  however,  of  division  of  effer- 
ent nerves,  if  the  peripheral  end  be  irritated,  thus  roughly 
counterfeiting  normal  stimulation,  the  ordinary  effects  of 
normal  stimulation  will  be  brought  about,  provided  (as  is 
usually  the  case)  that  particular  nerve  can  be  thus  directly 
stimulated.  Stimulation,  however,  of  the  central  end  of 
such  a  cut  nerve  produces  no  effect.  No  matter  whether 
such  efferent  nerves  receive  their  stimulus  directly  from  the 
center  or  artificially,  as  by  mechanical  or  electrical  means, 
the  effect  is  produced  in  the  end  organs,  whatever  they  may 


232  THE  NERVOUS  SYSTEM 

be.  It  is  an  invariable  law  to  which  reference  has  already 
been  made,  that  a  nerve  fiber  thus  conducting  a  message  in 
either  direction  is  not  interfered  with  by  the  proximity  of 
other  fibers,  similar  or  dissimilar.  Such  message  is  not  in 
any  way  imparted  to  a  neighboring  fiber  or  diffused  through 
the  fasciculus,  but  is  conveyed  uninterruptedly  to  its  destina- 
tion. It  is  possible  that  the  myeline  sheath  has  an  insulating 
effect  upon  the  contained  axis  cylinder,  just  as  an  electric 
wire  may  be  insulated  by  non-conducting  substances  like 
silk,  but  this  is  doubtful. 

Interesting  manifestations  of  motor  centrifugal  impulses 
are  seen  in  certain  movements  associated  with  correspond- 
ing muscles  on  different  sides  of  the  body  and  with  sets  of 
muscles  on  the  same  side.  It  is  almost  impossible  to  effect 
certain  movements  with  a  single  finger  or  toe  without  causing 
similar  movements  in  other  fingers  and  toes;  a  part  of  a 
muscle  cannot  be  made  to  contract  separately ;  it  is  doubtful 
if  it  be  possible  to  move  one  eye-ball  without  the  other,  even 
by  the  most  persistent  practice.  Other  similar  examples  are 
numerous.  It  is  quite  probable  that  in  most  cases  these  as- 
sociated movements  are  solely  matters  of  habit.  But  the 
connection  by  commissural  fibers  of  the  cells  in  the  centers 
controlling  and  regulating  the  movement  of  these  muscles 
and  sets  of  muscles  would  offer  a  not  unreasonable  explana- 
tion of  the  phenomena  in  question,  since  such  an  arrange- 
ment might  render  impossible  separate  and  individual  action 
by  the  cells  thus  connected.  Excepting,  perhaps,  the  move- 
ments of  the  eye-balls,  these  associated  movements  can  be 
greatly  modified  by  education. 

Characteristics  of  Afferent  Nerves. — Impressions  received 
by  these  fibers,  although  they  are  conveyed  toward  the  cen- 
ter and  must  reach  a  center  before  there  is  any  nervous 
manifestation,  are  always  referred  to  the  periphery.  A  most 
common  illustration  of  this  fact  is  furnished  by  injury  to  the 
ulnar  nerve  as  it  passes  the  elbow — such  injury  being  mani- 
fested not  usually  by  any  pain  at  the  point  of  infliction,  but  on 


AFFERENT    NERVES  233 

the  ulnar  side  of  the  hand  where  the  nerve  is  distributed.  A 
person  whose  limb  has  been  amputated  often  seems  to  feels 
pain  in  the  extremity  although  it  has  been  removed  from  the 
body — such  pain  coming  from  compression  by  the  cicatrix 
(or  otherwise)  of  the  nerves  which  before  the  amputation 
were  distributed  to  the  severed  limb.  Htere,  as  in  the  case  of 
efferent  nerves,  division  of  the  fibers  between  the  seat  of  im- 
pression and  the  center  precludes  the  possibility  of  any  ner- 
vous manifestation.  That  is  to  say,  no  pain  will  be  felt,  no 
matter  how  great  the  injury  be,  if  the  sensory  fibers  running 
from  the  seat  of  injury  be  divided.  Stimulation  of  the  peri- 
pheral end  of  a  divided  afferent  fiber  produces  no  effect; 
but  stimulation  of  the  central  end  is  followed  by  the  ordi- 
nary manifestation — by  pain  if  the  nerve  stimulated  be  a 
common  sensory  one.  This  remark,  of  course,  applies  only 
to  those  nerves  which  can  be  thus  directly  stimulated — 
typically  to  true  sensory  fibers. 

Impressions  conveyed  by  nerves  of  special  sense  must  be 
received  through  the  intervention  of  certain  complex  or- 
gans, consideration  of  which  belongs  elsewhere. 

Although  a  division  has  been  made  of  nerve  fibers  into 
afferent  and  efferent,  each  with  definite,  proper  and  dissim- 
ilar functions  so  far  as  the  direction  of  conduction  is  con- 
cerned, it  has  been  impossible  to  discover  any  actual  differ- 
ence in  the  composition,  appearance,  or  other  properties,  of 
the  actual  fibers  themselves.  In  fact,  it  may  be  even  consid- 
ered as  only  an  accident  that  one  fiber  conveys  a  message 
peripherially  and  another  centrally — an  accident  dependent 
upon  the  kind  of  center  with  which  the  fiber  is  connected 
and  the  kind  of  termination  it  has  in  the  periphery. 

Direction  of  the  Current  in  Nerve  Fibers. — It  has  long 
been  understood  that  in  no  case  will  a  fiber  in  situ  convey 
a  message  at  one  time  in  one  direction  and  at  another  in  an 
opposite  one,  that  no  individual  fiber  can  be  both  afferent 
and  efferent;  and  so  far  as  practical  action  is  concerned 
this  is  true,  but  "experiment  has  shown  that  if  a  nerve 


234  THE  NERVOUS  SYSTEM 

trunk  be  stimulated  at  a  given  point,  then  the  nerve  impulse 
can  be  demonstrated  as  passing  away  from  the  point  of 
stimulation  in  both  directions"  (American  Text-book). 
However,  only  the  message  traveling  in  the  physiological  di- 
rection is  manifest,  for  it  is  the  only  one  which  finds  a  suit- 
able terminal. 

It  is  not  to  be  concluded,  however,  that  in  any  nerve 
trunk,  as  the  ulnar  nerve,  there  may  not  be  both  afferent  and 
efferent  fibers.  Such,  in  fact,  is  the  usual  arrangement.  Any 
nerve  trunk  may  contain  all  kinds  of  fibers — sensory,  spe- 
cial sensory,  vaso-motor,  motor,  trophic,  secretory — but  the 
presence  of  all  these  does  not  interfere  with  the  individu- 
ality and  the  individual  action  of  each  fiber.  A  nerve  trunk 
containing  more  than  one  kind  of  fibers  is  called  a  mixed 
nerve. 

Speed  of  Nervous  Conduction. — It  is  stated  that  afferent 
impressions  are  conveyed  by  nerves  at  the  rate  of  about  120 
feet  per  second;  the  rate  for  efferent  impulses  is  somewhat 
less  rapid,  probably  no  feet.  In  the  spinal  cord  tactile  im- 
pressions are  conveyed  a  little  faster  than  in  the  nerves 
proper,  and  painful  impressions  somewhat  less  than  one- 
half  as  fast.  The  rate  of  motor  conduction  in  the  cord  is 
said  to  be  one-third  the  rate  in  the  nerves.  It  has  also  been 
demonstrated  that  an  act  of  volition  requires  a  definite 
time  for  the  inception  of  its  performance;  this  is  stated  to 
be  about  ^s  of  a  second.  The  recognition  of  a  simple  im- 
pression (conveyed  in  the  opposite  direction,  of  course)  re- 
quires about  ^5  of  a  second.  Furthermore,  the  part  played 
by  the  spinal  cord  in  reflex  action  (to  be  considered  later) 
also  consumes  an  appreciable  period;  this  is  found  to  be 
more  than  twelve  times  the  period  occupied  in  the  transmis- 
sion of  the  impression  to  the  cord  or  the  impulse  back  to  the 
muscles.  ' 

Action  of  Electricity  Upon  Nerves. — A  nerve  may  be  irri- 
tated in  any  one  of  several  ways;  but  mechanical,  thermal 
and  chemical  irritants,  besides  working  injury  to  the  tissues, 


THE   CEREBRO-S FINAL   AXIS  235 

are  much  less  easily  managed  and  regulated  than  is  elec- 
tricity. This  agent  may  be  applied  time  after  time  to  a  nerve 
trunk  without  causing  any  permanent  change  in  its  conduc- 
tivity, and  the  strength,  time  and  duration  of  application, 
etc.,  can  be  accurately  governed. 

It  has  been  noticed  that  the  uninterrupted  flow  of  an  elec- 
tric current  through  a  nerve  is  unattended  by  muscular  con- 
traction; it  has  likewise  been  seen  that  very  slow  changes 
in  the  strength  of  the  current  are  similarly  unaccompanied 
by  the  manifestations  of  ordinary  stimulation;  but  sudden 
changes  in  the  strength,  whether  in  the  direction  of  increase 
or  decrease,  act  as  stimuli.  However,  while  the  passage  of 
a  constant  current  through  a  nerve  does  not  manifest  itself 
by  contractions  except  at  making  and  breaking,  such  a-  pas- 
sage brings  about  a  change  in  the  tissue  of  the  nerve  known 
as  electrotonus.  It  may  be  considered  a  state  of  electric 
tension.  In  the  anodic  area  the  excitability  is  diminished 
(anelectrotonus)  ;  in  the  kathodic  area-  it  is  increased  (katel- 
ectrotonus).  Nor  is  the  electrotonic  condition  restricted  to 
that  portion  of  the  nerve  between  the  poles.  Between  the 
poles  there  is  a  point  where  the  two  influences — anelectro- 
tonus and  katelectrotonus — meet  and  there  is  neither  in- 
creased nor  decreased  excitability.  With  weak  currents  this 
point  is  nearer  the  anode ;  with  strong  ones  nearer  the  ka- 
thode. A  descending  current  diminishes  the  excitability  of 
a  nerve;  an  ascending  increases  it.  Prolonged  application 
of  electric  stimuli  will  exhaust  nervous  excitability,  but  it 
may  be  restored  by  rest,  or  more  quickly  by  an  opposite  cur- 
rent. 

THE   CEREBRO-SPINAL  AXIS. 

The  cerebro-spinal  axis  embraces  the  nervous  matter  in 
the  cranial  cavity  and  in  the  spinal  canal,  excepting  the  roots 
of  the  cranial  and  spinal  nerves.  This  axis  consists  of  both 
white  and  gray  matter.  The  white  matter  is  made  up  of 
conducting  elements ;  the  gray  matter  consists  of  a  number 


236  THE  NERVOUS  SYSTEM 

of  connected  ganglia.  In  the  cord  the  white  matter  is  situ- 
ated externally;  in  the  brain  the  gray.  The  encephalon  is 
situated  in  the  cranial  cavity  and  consists  of  the  cerebrum, 
the  cerebellum,  the  pons  Varolii,  and  the  medulla  oblongata. 
These  different  parts  are  connected  with  each  other  and 
with  the  cord  by  nerve  fibers,  and  all  the  cranial  and  spinal 
nerves  are  connected  with  gray  matter  either  in  the  brain  or 
in  the  cord,  or  in  both.  This  gray  matter  exists  for  the  pur- 
pose of  receiving  impressions  and  generating  nerve  force. 

Membranes. — The  encephalon  and  cord  are  covered  by 
membranes  for  protection  and  for  the  support  of  vessels  be- 
longing thereto.  These  are  (i)  the  dura  mater,  (2)  the  ar- 
achnoid, and  (3)  the  pia  mater. 

The  dura  mater  is  a  dense  fibrous  structure  surrounding 
the  encephalon  and  adherent  to  the  inner  surfaces  of  the 
cranial  bones.  At  certain  points  .the  two  layers  of  which  it 
is  composed  separate  to  form  the  venous  sinuses.  Processes 
of  the  internal  layers  also  are  sent  inward  between  the  two 
lobes  of  the  cerebrum  (falx  cerebri),  between  the  cerebrum 
and  cerebellum  (tentorium  cerebelli)  and  between  the  lateral 
halves  of  the  cerebellum  (falx  cerebelli).  This  membrane 
passes  through  the  foramen  magnum  to  cover  also  the  spinal 
cord,  and  to  follow  as  a  sheath  the  spinal  nerves  at  their 
foramina  of  exit. 

The  arachnoid  resembles  the  serous  membranes:  It  cov- 
ers the  brain  and  cord  underneath  the  dura' mater  without 
dipping  into  the  sulci  of  the  brain.  Between  it  and  the  pia 
mater  is  what  is  known  as  the  subarachnoid  space  containing 
the  subarachnoid  fluid.  This  fluid  serves  a  mechanical  pur- 
pose, equalizing  pressure  in  different  parts  of  the  cerebro- 
spinal  axis  and  protecting  the  nervous  substance  from  in- 
jury by  concussion,  etc.  Besides  being  found  in  the  subarach- 
noid space,  it  occupies  the  ventricles  of  the  brain  and  the 
central  canal  of  the  cord,  communication  between  these  being 
furnished  by  a  small  opening  at  the  inferior  angle  of  the 
floor  of  the  fourth  ventricle. 


THE  PIA  MATER 


237 


The  pia  mater  is  a  very  delicate  structure  dipping  between 
the  convolutions  of  nervous  matter  and  lying  in  close  con- 
tact with  the  external  surface  of  the  encephalon  and  cord. 
It  is  exceedingly  vascular,;  indeed  its  main  function  is  to 
support  vessels  belonging  to  the  nervous  substance  under- 
neath. Both  the  arachnoid  and  the  pia  mater  pass  out  at 
the  foramen  magnum  with  the  dura  to  cover  the  cord. 


PIG    73. — .Different  views  of  a  portion  of  the  spinal  cord  from  the 
cervical  region,  with  the  roots  of  the  nerves.     (Slightly  enlarged.) 


In  A,  the  anterior  surface  of  the  specimen  is  shown;  the  anterior  nerve-root 
of  its  right  side  is  divided;  in  B,  a  view  of  the  right  side  is  given;  in  C,  the 
upper  surface  is  shown;  in  D,  the  nerve-roots  and  ganglion  are  shown  from 
below,  i,  the  anterior  median  fissure;  2,  posterior  median  fissure;  3,  anterior 
lateral  depression,  over  which  the  anterior  nerve-roots  are  seen  to  spread;  4, 
posterior  lateral  groove,  int9  which  the  posterior  roots  are  seen  to  sink;  5,  ante- 
rior roots  passing  the  ganglion;  5',  in  A,  the  anterior  root  divided;  6,  the  poste- 
rior roots,  the  fibers  of  which  pass  into  the  ganglion  6';  7,  the  united  or  com- 
pound nerve;  7',  the  posterior  primary  branch,  seen  in  A  and  D  to  be  derived 
in  part  from  the  anterior  and  in  part  from  the  posterior  root.  (Kirkes  after 
Allen  Thomson.) 


238  THE  NERVOUS  SYSTEM 

The  Spinal  Cord. 

The  spinal  cord  occupies  the  spinal  canal  and  is  about 
eighteen  inches  long,  extending  from  the  foramen  magnum 
to  the  lower  border  of  the  first  lumbar  vertebra.  Its  distal 
extremity  is  in  the  shape  of  a  slender  filament  known  as 
iilum  terminate,  which  is  gray  in  color.  The  sacral  and 
coccygeal  nerves,  having  taken  origin  from  the  cord  in  the 
dorsal  region,  pass  downward  in  the  canal  to  find  exit 
through  the  sacral  and  coccygeal  foramina.  This  collection 
of  nerves  thus  passing  down  is  known  as  the  cauda  equina. 

Gross  Divisions  of  the  Spinal  Cord  in  Section. — Cross  sec- 
tion of  the  cord  reveals  the  division  of  its  substance  into 
two  lateral  halves  connected  by  the  anterior  and  posterior 
commissures.  In  the  center  of  the  cord,  and  between  these 
commissures,  is  a  small  opening,  the  central  canal  of  the 
cord,  communicating  with  the  fourth  ventricle  above.  This 
division  of  the  substance  of  the  cord  into  lateral  halves  is 
effected  by  the  two  median  fissures,  anterior  and  posterior. 
The  former  is  the  more  clearly  marked,  and  is  lined  through- 
out with  pia  mater.  It  is  bounded  posteriorly  by  the  an- 
terior white  commissure.  The  posterior  median  fissure  is 
not  lined  with  pia  mater  and  extends  anteriorly  as  far  as  the 
posterior  gray  commissure.  It  is  to  be  noted  that  there  are 
both  anterior  and  posterior  gray  commissures,  but  only  one 
white  commissure  (anterior),  which  is  bounded  posteriorly 
by  the  anterior  gray  commissure. 

Besides  the  anterior  and  posterior  median  fissures  there 
are  also  on  each  side  antero-lateral  and  postero-lateral  fis- 
sures, marking  the  lines  of  exit  of  the  anterior  and  posterior 
roots  of  the  spinal  nerves.  These  are  not  well  defined. 
They  divide  the  cord  into  anterior,  posterior  and  two  lateral 
columns. 

Arrangement  of  Gray  Substance. — The  disposition  of  the 
gray  substance  in  the  cord  (in  transverse  section)  is  some- 
what after  the  manner  of  the  letter  H,  each  lateral  portion 


THE  SPINAL  CORD  239 

representing  the  anterior  and  posterior  cornua  of  gray  mat- 
ter for  that  side,  and  being  connected  to  the  corresponding 
portion  of  the  other  side  by  the  commissures  embracing  the 
central  canal.  The  anterior  cornua  are  shorter  and  thicker 
than  the  posterior.  From  these  issue  the  anterior  and  pos- 
terior roots  respectively  of  the  spinal  nerves.  The  cells  are : 
(i)  Those  in  the  anterior  cornu;  (2)  those  in  the  posterior 
cornu;  (3)  those  in  the  lateral  aspect  of  the  gray  matter; 
(4)  those  at  the  inner  base  of  the  posterior  cornu  (Clarke's 
vesicular  column). 

The  gray  substance  is  made  up  of  cells  with,  of  course, 
the  usual  neuroglia  and  blood-vessels.  The  cells  in  the  an- 
terior cornua  are  large  in  size  and  possess  a  greater  number 
of  poles  than  those  in  the  posterior  cornua ;  from  their  con- 
nection with  the  anterior  (motor)  spinal  nerve  roots  they 
are  called  motor  cells  in  contradistinction  to  the  sensory 
cells  in  the  posterior  cornua  which  are  connected  indirectly 
with  the  posterior  (sensory)  nerve  roots. 

Degeneration. — Nerve  fibers  when  separated  from  the 
cells  of  which  they  are  outgrowths  degenerate.  Fibers  have 
been  said  to  degenerate  in  the  direction  in  which  they  carry 
messages,  but  this  is  by  no  means  always  so.  For  instance, 
the  parent  cells  for  the  fibers  of  the  posterior  spinal  roots 
are  in  the  ganglia  on  those  roots  near  the  cord,  and  section 
of  the  root  beyond  the  ganglion  causes  degeneration  of  its 
fibers  peripherally — which  is  in  the  opposite  direction  to  the 
passage  of  impressions  in  them.  Section  of  the  posterior 
root  between  the  ganglion  and  cord  is  followed  by  centripetal 
degeneration,  and  there  is  no  centrifugal  degeneration.  The 
anterior  spinal  root  fibers  are  outgrowths  of  cells  in  the  an- 
terior cornua  of  gray  matter.  Section  of  this  root  anywhere 
occasions  centrifugal  degeneration  (Fig.  74). 

Arrangement  of  the  White  Substance. — It  is  scarcely 
necessary  to  state  that  the  white  substance  of  the  cord  con- 
sists of  nerve  fibers  with  their  usual  accompaniments.  It  is 
external  to  the  gray.  The  fibers  are  medullated,  but  have 
no  sheath  of  Schwann. 


240 


THE  NERVOUS  SYSTEM 


The  divisions  of  the  cord  already  referred  to  are  purely 
anatomical.  Physiological  and  pathological  researches  war- 
rant the  further  division  of  the  white  substance  of  the  cord 
into  eight  columns  for  each  side.  The  course  of  all  the  fibers 
in  the  white  matter  of  the  cord  is  by  no  means  certain.  The 
division  here  given  may  not  be  strictly  correct,  but  it  prob- 


FIG.  74. — Diagram  to  illustrate  wallerian  degeneration  of  nerve- 
roots.     (Kirkes.) 

ably  receives  as  little  adverse  criticism  as  any  of  the  others. 
Classified  according  to  the  direction  in  which  their  fibers 
degenerate  after  section  the  paths  are:  (I)  Degenerating 
downward,  (a)  the  column  of  Turck  and  (b)  the  crossed 
pyramidal  tract;  (II)  degenerating  upward,  (a)  the  col- 
umn of  Goll,  and  (b)  the  direct  cerebellar  tract;  (III)  de- 
generating in  neither  direction,  (a)  the  anterior  fundamental 
fasciculus,  (b)  the  anterior  radicular  zone,  (c)  the  mixed 
lateral  column,  and  (d)  the  column  of  Burdach. 

I.  (a)  The  column  of  Turck  occupies  a  position  just  lat- 
eral to  the  anterior  median  fissure  and  extends  downward  to 
the  lower  dorsal  region.  Its  fibers  decussate  high  up  in  the 
cord.  This  column  is  sometimes  called  the  direct,  or  un- 


THE  SPINAL  CORD  24! 

crossed,  pyramidal  tract,  as  distinguishing  it  from  the  other 
descending  column,  (b)  The  crossed  pyramidal  tract  is  ex- 
ternal to  the  posterior  cornu  of  gray  matter  and  internal  to 
the  direct  cerebellar  tract.  Its  fibers  decussate  in  the  an- 
terior pyramids  of  the  medulla  oblongata. 

II.  (a)  The  direct  cerebellar  tract  occupies  the  outer  pos- 
terior part  of  the  lateral  column.     Its  fibers  reach  the  cere- 

b 


...Ji 


FIG.  75. — Scheme  of  the  conducting  paths  in  the  spinal  cord  at  the 
third  dorsal  nerve. 

The  black. part  is  the  gray  matter,  v,  anterior,  hw,  posterior  root;  a,  direct, 
and  g,  crossed,  pyramidal  tracts;  b,  anterior  fundamental  fasciculus;  c,  Goll  s 
column;  d,  column  of  Burdach;  e,  anterior  radicular  zone;  /,  mixed  lateral  tract; 
h,  direct  cerebellar  tracts.  (Landois,  modified.) 

bellum  through  the  inferior  peduncles,  after  having  trav- 
ersed the  posterior  pyramids  of  the  medulla.  This  tract 
exists  throughout  the  length  of  the  cord,  (b)  The  column 
of  Goll  (postero-internal  column)  is  situated  posteriorly  in 
a  position  corresponding  to  the  column  of  Turck  anteriorly 
— just  lateral  to  the  posterior  median  fissure.  Fibers  in  this 
column  extend  from  the  upper  lumbar  region  to  the  funi- 
culi  graciles  of  the  medulla. 

III.  (a)  The  anterior  fundamental  fasciculus  lies  between 

the  column  of  Turck  internally  and  the  anterior  cornu  and 

anterior  roots  of  the  spinal  nerves  externally.    Its  fibers  are 

lost   in 'the   medulla   above.      (b)    The   anterior  radicular 

16 


242 


THE  NERVOUS  SYSTEM 


zone  is  external  to  the  anterior  roots  of  the  spinal  nerves 
and  anterior  to  the  crossed  pyramidal  tract  and  the  direct 


FIG.  76. — Course  of  the  fibers  for  voluntary  movement. 

ab,  path  for  the  motor  nerves  of  the  trunk;  c,  fibers  of  the  facial  nerve;  B, 
corpus  callosum;  Nc,  nucleus  caudatus;  Gi,  internal  capsule;  N,  I,  lenticular 
nucleus;  P,  pons;  N.  f.,  origin  of  the  facial;  Py,  pyramids  and  their*  dccussa- 
tion;  Ol,  olive;  Gr,  restiform  body;  P.R.,  posterior  root;  A.R.,  anterior  root;  x, 
crossed,  and  s,  direct  pyramidal  tracts.  (Landois.) 

cerebellar    fasciculus.      Its   fibers   are   lost   in   the   medulla 
above,     (c)  The  mixed  lateral  column  is  just  external  to  the 


MOTOR    PATHS    IN    THE    CORD  243 

main  body  of  gray  matter  and  does  not  reach  the  surface  of 
the  cord.  Its  fibers  are  likewise  lost  in  the  medulla  ob- 
longata.  (d)  The'  column  of  Burdach  (postero-external 
column)  is  situated  posteriorly  in  a  location  corresponding 
to  the  anterior  fundamental  fasciculus  anteriorly — external 
to  the  column  of  -Goll  and  internal  to  the  posterior  cornu. 
Its  fibers  reach  the  cerebellum  through  the  inferior  pe- 
duncles, having  passed  through  the  restiform  bodies. 

Functions  of  the  Columns. — Remarks  already  made  touch- 
ing the  direction  of  degeneration  in  the  separate  columns 
throw  some  light  upon  the  physiological  function  of  the 
fibers  in  each. 

Motor  impulses  pass  downward  from  the  brain  through 
certain  fibers  to  the  cells  of  the  anterior  cornua  of  gray  mat- 
ter in  the  cord,  and  are  sent  thence  through  the  spinal  nerves 
to  the  muscles.  The  paths  in  the  cord  conveying  these  im- 
pulses are  found  to  be  the  columns  of  Turck  and  the  crossed 
pyramidal  tracts,  and  these  are  the  only  parts  of  the  cord 
known  so  to  act.  Impulses  to  the  upper  segment  of  the  cord 
may  be  conveyed  by  either  of  these  columns,  but  impulses  to 
the  lower  segment  must  follow  the  crossed  pyramidal  tract, 
since  the  column  of  Turck  ceases  to  exist  in  the  dorsal  re- 
gion. Only  some  3-7  per  cent,  of  motor  fibers  from  the  cor- 
tex are  thought  to  enter  the  columns  of  Turck.  The  others 
decussate  in  the  medulla  and  enter  the  crossed  pyramidal 
tracts.  In  any  case  motor  impulses  originating  in  the  brain 
and  so  conveyed  are  manifested  on  the  side  opposite  their 
cerebral  origin,  since  the  fibers  in  both  these  tracts  decussate 
in  passing  downward.  It  is  a  well  known  pathological  fact 
that  the  lesions  of  motor  areas  in  the  brain,  or  section  of  one 
lateral  half  of  the  cord,  are  followed  by  paralysis  on  the  side 
opposite  the  lesion. 

Following  a  motor  fiber  (A,  Fig.  77)  through  the  anterior 
root  of  a  spinal  nerve,  it  is  found  to  originate  from  one  of 
the  large  multipolar  cells  (3)  in  the  anterior  cornu  of  gray 
matter.  Around  these  anterior  horn  cells  (i,  2,  3,  4)  arbor- 


244 


THE  NERVOUS  SYSTEM 


ize  the  end  filaments  of  fibers  which  have  come  down 
through  the  cord  from  the  brain.  Some  fibers  have  come 
down  in  the  uncrossed  pyramidal  tract  (column  of  Turck) 
on  the  side  opposite  the  cells,  i,  2,  3,  4,  and  crossed  over  to 
the  same  side  through  the  anterior  white  commissure  ap- 


FIG.  77. — Course  of  nerve  fibers  in  spinal  cord.     (Kirkes  after 
S  chafer.) 

proximately  on  a  level  with  the  cells ;  others  have  decussated 
in  the  medulla,  and  come  down  in  the  crossed  pyramidal 
tract  on  the  same  side  as  the  cells.  In  both  cases  the  fibers 
originated  in  the  brain  on  the  side  opposite  the  cells  around 
which  they  arborize  in  the  cord.  This  is  the  connection 
which  exists  between  the  brain  and  the  anterior  root  fibers. 
Not  all  fibers  in  the  anterior  nerve  roots  are  thus  pro- 


SENSORY  PATHS  IN  THE  CORD  245 

longed  upward  in  the  pyramidal  tracts.  The  number  of 
fibers  in  these  roots  is  much  larger  than  in  the  pyramidal 
tracts,  and  consequently  some  of  them  must  end  (originate) 
directly  in  the  cells  of  the  anterior  cornua.  Furthermore,  it 
seems  that  some  fibers  pass  from  the  anterior  nerve  roots 
directly  into  the  pyramidal  tracts  without  being  interrupted 
by  motor  cells. 

The  column  of  Turck  and  the  crossed  pyramidal  tract 
are,  therefore,  the  motor  paths  in  the  cord. 

Fibers  entering  the  cord  by  the  posterior  roots  send  pro- 
longations both  upward  and  downward  in  the  gray  matter  of 
the  cord,  and  communicate  by  end  arborizations  with  the 
small  sensory  cells  in  the  posterior  cornua  and  with  cells  in 
several  other  localities.  (See  Figs.  77,  84.)  Reference  to 
Fig.  77  will  show  that  the  connection  of  the  anterior  nerve 
fibers  with  the  gray  matter  of  the  cord  is  simple,  while  that 
of  the  posterior  is  comparatively  complex,  i,  2,  3,  4  are  an- 
terior horn  cells.  Each  of  these  gives  rise  to  an  efferent 
fiber,  one  of  which  (A)  is  shown  distributed  to  a  muscle 
(M).  Each  of  these  cells  also  is  surrounded  by  the  end  ar- 
borization of  a  fiber  (P)  from  the  cortex. 

A  fiber  from  the  posterior  root  is  also  shown.  It  origin- 
ates in  a  cell  of  the  sensory  ganglion  (G).  It  bifurcates, 
one  branch  going  to  the  surface  (S),  the  other  enters  the 
cord  and  itself  bifurcates.  The  branch  (E)  is  short  and 
arborizes  around  a  small  cell  (Pi)  in  the  posterior  cornu, 
from  which  a  new  axis  cylinder  arises  to  arborize  around 
the  anterior  horn  cell  (4).  The  other  branch  (D)  travels 
upward  in  the  posterior  column  of  the  cord.  A  collateral  (5) 
is  seen  going  to  the  anterior  horn  cell  (2),  one  to  the  pos- 
terior horn  cell  (P2)  and  another  to  a  cell  (C)  in  the  inner 
base  of  the  posterior  cornu  (in  Clarke's  column)  ;  from  C 
an  axis  cylinder  enters  the  direct  cerebellar  tract.  The 
main  fiber  (8)  may  terminate  in  the  gray  matter  of  the 
cord  above,  or  in  the  medulla.  Impressions  brought  thus  to 
the  cord  are  carried  to  the  opposite  side  and  pass  up 


246 


THE  NERVOUS  SYSTEM 


through  the  gray  matter  in  most  part.  The  fibers  decussate 
at  no  particular  point,  but  throughout  the  length  of  the  cord. 
However,  some  fibers  bearing  sensory  impressions  pass  to 
the  column  of  Goll  and  thus  upward,  while  some  also  go  to 


FIG.  78. — Transverse  section  through  half  the  spinal  cord,  showing 

the  ganglia. 

A,  anterior  cornuaJ  cells;  B,  axis-cylinder  process  of  one  of  these  going  to 
posterior  root;  C,  anterior  (motor)  root;  D,  posterior  (sensory)  root;  E,  spinal 
ganglion  on  'posterior  root;  F,  sympathetic  ganglion;  G,  ramus  communicans; 
H,  posterior  branch  of  spinal  nerve;  /,  anterior  branch  of  spinal  nerve;  a,  long 
collaterals  from  posterior  root  fibers  reaching  to  anterior  horn;  b,  short  collater- 
als passing  to  Clarke's  column;  c,  cell  in  Clarke's  column  sending  an  axis-cylin- 
der process  (d)  to  the  direct  cerebellar  tract;  e,  fiber  of  the  anterior  root;  f, 
axis  cylinder  from  sympathetic  ganglion  cell,  dividing  into  two  branches,  one 
to  the  periphery,  the  other  toward  the  cord;  g,  fiber  of  the  anterior  root  termi- 
nating by  an  arborization  in  the  sympathetic  ganglion;  h,  sympathetic  fiber  pass- 
ing to  periphery.  (Kirkes  after  Romany  Cajal.) 


the  encephalon  by  the  direct  cerebellar  fasciculi  and  the  col- 
umns of  Burdach.  Experimentally,  decussation  of  sensory 
fibers  is  demonstrated  (i)  by  longitudinal  section  of  the 
spinal  cord  in  the  median  line,  which  is  followed  by  anes- 


THE  COLUMNS   OF   BURDACH  247 

thesia  on  both  sides  below  the  section;  and  (2)  by  horizontal 
section  of  one-half  of  the  cord,  which  is  followed  by  anes- 
thesia on  the  opposite  side  below  the  section.  It  is  claimed 
that  pain  and  temperature  sensations  decussate  at  once  on 
reaching  the  gray  matter,  while  sensations  of  touch,  pres- 
sure and  equilibration  pass  up  on  the  same  side  until  the  me- 
dulla is  reached.  Some  afferent  fibers  are  probably  not  con- 
tinued upward  to  the  brain  either  directly  or  indirectly. 

It  thus  appears  that  we  have  no  very  accurate  knowledge 
of  the  sensory  paths  in  the  cord.  The  gray  matter  seems 
principally  concerned ;  but  the  columns  of  Goll  and  Burdach 
and  the  direct  cerebellar  fasciculi  also  convey  afferent  im- 
pressions. 

The  columns  of  Burdach  have  been  said  to  present  no  de- 
generation secondary  to  section.  Trophic  centers  for  their 
fibers  must,  therefore,  exist  above  and  below  any  given  point 
of  section.  It  is  found  that  the  fibers  constituting  these  col- 
umns pass  in  and  out  along  the  cord  between  cells  in  differ- 
ent planes  and  acting  as  longitudinal  commissural  fibers.  In 
locomotor  ataxia  the  characteristic  symptom  is  inability  to 
coordinate  the  muscular  movements — especially  of  the  lower 
extremities ;  the  characteristic  lesion  has  been  found  to  be  in 
the  columns  of  Burdach.  This  is  of  importance  in  deter- 
mining the  function  of  these  columns,  and,  in  fact,  leads  to 
the  conclusion  that  their  fibers  assist  in  regulating  and  co- 
ordinating the  voluntary  movements.  This  opinion  is  fur- 
ther supported  by  the  connection  of  these  fibers  with  the 
cerebellum,  which  contains  the  center  for  muscular  coordi- 
nation— if  such  a  center  exist.  The  sense  of  pressure  and 
the  so-called  muscular  sense  are  probably  connected  with  the 
fibers  of  this  column,  and  these  may  be  the  only  sensory  im- 
pressions conveyed  through  the  columns  of  Burdach. 

The  anterior  fundamental  fasciculi,  the  anterior  radicular 
zones,  and  the  mixed  lateral  paths  degenerate  in  neither 
direction  after  section,  their  trophic  cells  existing  at  both  ex- 
tremities. They  connect  cells  in  the  gray  matter  of  the  cord. 


243  THE  NERVOUS  SYSTEM 

Functions  of  the  Spinal  Cord. — These  are  (i)  conduc- 
tions, (2)  transference,  (3)  reflex  action,  (4)  augmenta- 
tion, (5)  coordination,  (6)  inhibition  of  reflex  acts,  (7) 
special  centers  (Collin  and  Rockwell,  modified). 

i.*  Conduction. — This  has  been  referred  to  in  discussing 
the  white  columns  of  the  cord.  This  function  makes  it  pos- 
sible for  the  brain  to  receive  impressions  from  and  send  im- 
pulses to  the  periphery.  It  is  to  be  remembered  that  most  of 
these  impressions  and  impulses  are  interrupted  by  spinal 
nerve  cells  in  their  passage  between  brain  and  periphery. 

2.  Transference. — An  impression  reaching  the  gray  mat- 
ter of  the  cord  may  be  transferred  (not  as  in  typical  reflex 
action)  so  as  to  be  felt  in  an  entirely  different  region  from 
that  in  which  the  irritation  takes  place.     Hip  joint  disease 
often  gives  pain  in  the  knee  alone. 

3.  Renex  Action. — The  cord  may  act  as  a  center  without 
the  cooperation  of  the  brain.    Indeed,  by  no  means  do  mus- 
cular movements  cease  immediately  on  removal  of  the  en- 
cephalon  if  the  cord  and  its  nerves  be  left  intact.     An  ani- 
mal so  mutilated  possesses  no  sensation  or  volition,  but  for  a 
time  the  sensory  nerves  will  continue  to  convey  impressions 
and  the  motor  nerves  impulses.    Under  these  conditions  im- 
pressions (as  of  heat)  are  conveyed  to  the  cord  by  the  affer- 
ent nerves ;  the  gray  matter  of  the  cord  receives  the  impres- 
sions and  generates  motor  force  which  is  sent  out  through 
the  corresponding  efferent  nerves,  and  movements  result. 
This  is  reflex  action.     The  impression  is  reflected  through 
the  cord  and  manifested  in  motion  without  the  intervention 
of  sensation   or  volition.     Reference  to   Figs.   77  and   80 
shows  how  reflex  action  is  anatomically  possible  through  the 
cord  connections.     Typical  reflex  action  requires  anatomic- 
ally (i)  something  to  produce  an  impression,  (2)  a  nerve 
terminal  to  receive  it,  (3)   a  centripetal  fiber  to  convey  it, 
(4)  a  center  to  receive  and  transform  it,  (5)  a  centrifugal 
fiber  to  convey  it  to  the  periphery,  and  (6)  a  muscle  to  con- 
tract.   This  remark  applies  to  reflex  action  connected  with 


REFLEX  ACTION  249 

the  cord,  but  by  common  consent  reflex  action  is  not  limited 
to  the  cord  and  its  connections. 

If  reflex  action  be  defined  as  any  involuntary  manifestation 
of  nerve  force  consequent  upon  the  reception  of  an  impres- 
sion (general  or  special)  by  a  nerve  center,  the  term  must  be 
made  to  include  such  phenomena  as  intestinal  peristalsis, 
contraction  and  dilatation  of  the  pupil,  certain  mental  op- 
erations, etc.  In  reality  most  reflex  acts  are  of  a  complex 
nature,  involving  associated  action  on  the  part  of  several 
neurons  and  being  manifested  frequently  at  several  points. 
For  example,  a  foreign  body  in  the  larynx  causes  reflexly 
not  only  closure  of  the  glottis,  but  also  the  convulsive  mus- 
cular contractions  incident  to  coughing.  The  realm  of  reflex 
action  is  obviously  a  wide  one. 

It  may  be  said  that  ordinary  reflexes  are  usually  under  the 
direction  of  the  cord,  but  this  does  not  imply  that  the  brain 
may  not  be  concerned.  Pricking  the  sole  of  the  foot  of  a 
sleeping  person  will  cause  him  to  draw  up  his  leg  without 
the  intervention  of  consciousness.  Probably  were  he  awake 
the  withdrawal  would  still  be  a  reflex  but  he  would  certainly 
be  conscious  of  the  pain,  though  after  the  act  of  withdrawal 
zvas  accomplished.  Nor  is  reflex  action  by  any  means  lim- 
ited to  the  cerebro-spinal  system.  Either  of  the  two  sys- 
tems, or  both,  may  be  concerned. 

Now  in  order  for  reflex  movements  to  occur,  there  must 
be  a  transference  of  impressions  received  by  sensory  cells  to 
cells  capable  of  giving  origin  to  motor  impulses.  The  cells 
communicate  by  their  collaterals,  which  may  be  short  or 
long,  depending  on  the  distance  between  the  cells  concerned. 
Cells  in  the  gray  matter  of  the  cord  are  "connected"  by  such 
fibers,  and  they  run  largely  in  the  white  matter  of  the  cord 
joining  cells  on  different  planes.  They  constitute  the  larger 
part  of  the  anterior  fundamental  fasciculi,  the  anterior  radic- 
ular  zones,  and  the  mixed  lateral  tracts,  and  it  is  these  paths 
which  are  mainly  concerned  in  reftex  action  of  the  cord, 

4.  Augmentation.— Sensory  fibers,  on  reaching  the  cord, 


250  THE  NERVOUS  SYSTEM 

send  prolongations  both  upward  and  downward  in  the  gray 
matter.  These  prolongations,  by  their  end  arborizations, 
seem  to  communicate  indirectly  with  several  motor  cells.  In 
the  simplest  reflex  movements  connected  with  the  spinal  cord 
the  muscular  activity  is  limited  to  the  area  corresponding 
to  the  distribution  of  the  afferent  nerve  which  has  been  irri- 
tated; but  if  the  irritation  be  sufficiently  increased  other 
muscles  in  the  same  locality,  or  the  corresponding  muscles 
on  the  opposite  side  of  the  body,  or  even  the  whole  muscu- 
lature, may  be  thrown  into  action.  This  is  explained  on  the 
ground  that  under  favorable  conditions  of  central  excita- 
bility, strength  of  peripheral  irritation,  etc.,  the  afferent  im- 
pression is  disseminated  by  collaterals  throughout  a  large 
area  of  the  cord  (for  example),  and  a  large  number  of  effer- 
ent cells  are  made  to  discharge.  The  reflex  excitability  of 
the  cord  is  markedly  increased  by  the  administration  of 
such  drugs  as  strychnin.  An  animal  so  poisoned  will  be 
thrown  into  the  most  violent  convulsions  by  so  slight  a  sen- 
sory impression  as  a  simple  breath  of  air.  Removal  of  the 
encephalon  in  inferior  animals  also  exaggerates  reflex  ex- 
citability. 

5.  Coordination. — This  has  been   referred  to   under  the 
columns  of  Burdach.    Coordination  is  "a  repetition  of  ordi- 
nary reflex  acts  for  our  daily  lives."    No  effort  is  necessary 
to  coordinate  the  muscular  movements  of  deglutition,  res- 
piration, walking,  etc.    These  movements  may  be  performec^ 
when  the  cerebrum  is  removed. 

6.  Inhibition  of  Refiex  Acts. — This  is  not  a  function  of 
the  cord  proper,  but  is  directed  by  the  cerebrum.     A  great 
many  reflex  movements  may  be  inhibited  by  an  act  of  the 
will,  providing  always  they  are  due  to  contraction  of  striped 
muscle.     The  reflex  acts  of  coughing  or  sneezing,  or  those 
resulting   from  tickling,   for  example,  can  be  largely  con- 
trolled.    These  are  usually  performed  as  reflex  cord  acts, 
but  the  brain  may  evidently  assert  its  superiority  over  the 
cord  and  inhibit  them. 


THE  ENCEPHALON  25! 

7.  Special  Centers. — In  the  gray  matter  of  the  cord  are 
found  various  centers  for  distinct  acts  such  as  defecation, 
parturition,  micturition,  etc.  These  are  all  connected  with 
each  other  and  with  the  encephalon  and  obey  the  usual  laws 
of  reflex  action. 

THE  ENCEPHALON. 

The  encephalon  is  situated  within  the  cranial  cavity  and 
is  commonly  called. the  brain.  Its  gross  divisions  are  the 
medulla  oblongata,  the  pons  Varolii,  the  cerebellum,  and  the 
cerebrum.  All  the  other  divisions  are  in  a  measure  subordi- 
nate to  the  cerebrum,  though  each  division  has  individual 
functions.  The  human  brain  weighs  about  49^/2  ounces  in 
the  male  and  about  44  in  the  female. 

The  Medulla  Oblongata. 

Anatomy. — The  medulla  oblongata,  or  bulb,  joins  the 
upper  extremity  of  the  spinal  cord  and  extends  to  the  pons 
above.  It  has  a  pyramidal  shape,  lies  in  the  basilar  groove 
of  the  occipital  bone,  and  is  slightly  flattened  antero-poster- 
iorly.  It  is  about  an  inch  and  a  quarter  in  length,  half  an 
inch  thick,  and  three-quarters  of  an  inch  broad  above.  The 
anterior  and  posterior  median  fissures  of  the  cord  are  con- 
tirmed  upward  in  the  medulla ;  the  central  canal  terminates 
in  the  inferior  angle  of  the  fourth  ventricle.  The  anterior 
columns  appear  to  be  continuous  with  the  anterior  pyramids 
of  the  medulla.  These  pyramids  are  situated  just  lateral  to 
the  anterior  median  fissure.  The  innermost  fibers  of  the 
pyramids  are  the  continuations  upward  of  the  crossed  pyra- 
midal tracts,  and  are  seen  to  decussate  in  the  median  line ; 
the  outermost  fibers  are  the  prolongations  of  the  uncrossed 
pyramidal  tracts.  The  olivary  bodies,  oval  in  shape,  are 
just  external  to  the  anterior  pyramids  separated  from  them 
by  a  groove.  The  restiform  bodies  make  up  the  postero- 


252  THE   NERVOUS   SYSTEM 

lateral  portion  of  the  medulla,  and  are  external  to  the  oli- 
vary bodies.  They  contain  fibers  from  the  columns  of  .Bur- 
dach,  and  contribute  largely  to  the  formation  of  the  inferior 
peduncles  of  the  cerebellum.  The  restiform  bodies,  diverg- 
ing as  they  ascend,  form  the  lateral  boundaries  of  the  in- 
ferior division  of  the  fourth  ventricle.  Beneath  the  olivary 


FIG.  79. — Floor  of  the  4th  ventricle  and  the  connections  of  the 
cerebellum. 

On  the  left  side  the  three  cerebellar  peduncles  are  cut  short;  on  the  right  the 
connections  of  the  superior  and  inferior  peduncles  have  been  preserved,  while 
the  middle  one  has  been  cut  short,  i,  median  groove  of  the  4th  ventricle  with 
the  fasciculi  teretes;  2,  the  striae  of  the  auditory  nerve  on  each  side  emerging 
from  it;  3,  inferior  peduncle;  4,  posterior  pyramid  and  claya,  with  the  calamus 
scriptorius  above  it;  5,  superior  peduncle;  6,  fillet  to  the  side  of  the  crura  cer- 
ebri;  8,  corpora  quadrigemina.  (Landois.) 

bodies,  and  between  the  anterior  pyramids  and  the  restiform 
bodies,  are  the  lateral  fasciculi,  or  the  funiculi  of  Rolando. 
They  constitute  the  upward  prolongation  of  all  the  antero- 
lateral  portion  of  the  cord  which  does  not  go  to  the  forma- 
tion of  the  auterior  pyramids.  Their  chief  importance  is  in 
the  fact  that  they  contain  the  centers  for  respiration.  The 
posterior  pyramids  are  sometimes  called  the  funiculi  graciles. 
They  join  the  restiform  bodies  and  pass  to  the  cerebellum. 


THE  MEDULLA  OBLONGATA  253 

The  fourth  ventricle  deserves  particular  attention.  It  is 
a  cavity  on  the  posterior  aspect  of  the  pons  and  medulla  ex- 
tending from  the  upper  limit  of  the  former  to  a  point  on  the 
latter  opposite  the  lower  border  of  the  olivary  body.  It  has 
the  shape  of  two  isosceles  triangles  placed  base  to  base.  The 
apex  of  the  inferior  triangle  is  at  the  calamus  scriptorious, 
and  its  lateral  boundaries  are  the  diverging  restiform  bodies. 
The  superior  peduncles  of  the  cerebellum  form  the  lateral 
boundaries  of  the  superior  triangle.  The  inferior  triangle 
is  covered  by  the  cerebellum;  the  superior  by  the  valve  of 
Vieussens,  which  stretches  between  the  superior  peduncles. 
This  ventricle  communicates  above  with  the  third  ventricle 
by  the  aqueduct  of  Sylvius,  or  the  iter  a  tertio  ad  quartum 
ventriculum;  below,  with  the  central  canal  of  the  cord  and 
with  the  subarachnoid  space.  The  floor  of  the  ventricle 
presents  a  longitudinal  median  fissure  and  numerous  small 
elevations  indicating  the  position  of  the  nuclei  of  origin  of 
certain  of  the  cranial  nerves. 

The  gray  matter  of  the  medulla  has  the  same  general  dis- 
tribution as  that  in  the  cord,  but  is  by  no  means  so  regular  in 
its  disposition.  The  direction  of  the  white  fibers  is  not  so 
uniform  as  in  the  cord.  They  run  not  only  longitudinally,  but 
transversely  to  connect  the  lateral  halves,  and  in  other  direc- 
tions to  connect  various  centers  situated  in  this  part  of  the 
encephalon  and  to  connect  the  medulla  with  other  parts  of 
the  brain.  The  following  is  the  relation  of  the  columns  of 
the  cord  to  the  medulla : 

The  direct  and  crossed  pyramidal  tracts  pass  to  the  ence- 
phalon constituting,  in  the  medulla,  the  anterior  pyramids — 
the  direct,  having  decussated  below,  occupying  here  the  outer 
portion  of  the  pyramid,  and  the  crossed  decussating  in  the 
medulla  and  occupying  the  inner  portion  of  the  pyramid. 

Those  columns  concerned  in  reflex  action,  the  anterior 
fundamental  fasciculi,  the  anterior  root  zones  and  the  mixed 
lateral  tracts  do  not  continue  farther  upward  than  the  gray 
matter  of  the  medulla. 


254  THE   NERVOUS   SYSTEM 

The  columns  of  Coll  are  continuous  with  the  funiculi 
graciles. 

The  columns  of  Burdach  and  the  direct  cercbellar  fasci- 
culi pass  to  the  cerebellum  through  the  restiform  bodies  of 
the  medulla. 

Functions. — The  functions  of  the  medulla  are  (i)  con- 
duction, (2)  reflex  action,  (3)  to  furnish  centers  for  special 
acts. 

i.  As  a  conductor  the  medulla  is  absolutely  necessary  as  a 
means  of  connection  between  the  brain  and  the  cord.  Sen- 
sory impressions  to  and  motor  impulses  from  the  brain 
must  all  pass  through  by  this  route. 

As  a  reflex  nerve  center  the  medulla  also  resembles  the 
cord,  though  impressions  reflected  through  this  organ  are 
frequently  much  less  simple  than  those  reflected  through  the 
cord.  Reflex  action  in  the  medulla  is  dependent  on  (3),  to 
be  noticed  now. 

3.  The  most  important  center  presiding  over  coordinated 
movements  is  that  for  respiration.  The  encephalon  may  be 
cut  away  down  as  far  as  the  medulla,  and  life  will  continue 
for  a  certain  time.  It  is  also  true  that  the  medulla  itself  may 
be  gradually  cut  away  from  above  downward  until  a  certain 
point  is  reached,  when  respiration  suddenly  ceases.  Likewise 
the  spinal  cord  may  be  cut  away  upward  till  this  point  is 
reached,  when  the  same  results  will  follow.  This  is  the  true 
respiratory  center,  and  is  situated  at  the  site  of  origin  of  the 
vagi.  Its  destruction  is  followed  by  an  immediate  suspension 
of  respiration  and  consequent  death  by  asphyxia,  though 
there  is  no  manifestation  of  the  distress  usually  accompany- 
ing this  condition.  The  sense  of  want  of  air  is  simply  lost. 
There  is  one  of  these  centers  for  each  side,  but  they  act  syn- 
chronously, being  connected  by  commissural  fibers.  Probably 
the  usual  mode  of  stimulation  of  the  respiratory  center  is  by 
afferent  impressions,  but  it  may  also  be  stimulated  directly, 
as  by  deoxygenated  blood.  Mutilation  of  the  medulla,  on 
account  of  the  presence  of  this  center,  is  followed  by  the 


THE    PONS    VAROLIJ  255 

nearest  approach  to  instantaneous  death,  and  the  respiratory 
center  has,  therefore,  been  called  the  "vital  spot,"  though 
death  from  any  cause  cannot  be  instantaneous. 

Some  other  reflex  centers  are  for  deglutition,  sucking,  se- 
cretion of  saliva,  vomiting,  coughing,  sneezing,  dilatation  of 
the  pupil,  secretion  of  sweat,  secretion  of  glycogen,  etc. 
Typical  of  these  is  the  reflex  act  of  sneezing,  in  which  case 
impressions  are  conveyed  to  the  medulla  by  the  nasal 
branches  of  the  fifth  nerve. 

Additional  centers  in  the  medulla  are  those  which  preside 
over  inhibition  and  acceleration  of  the  heart,  vaso-motor 
centers  for  the  vessel  walls,  and  centers  for  special  senses 
like  hearing  and  taste.  There  is  also  said  to  be  here  a  center 
controlling  the  production  of  heat  by  the  tissues. 

The  Pons  Varolii. 

Anatomy. — The  pons  is  situated  just  above  the  medulla 
oblongata  at  the  base  of  the  brain,  and  is  frequently  called 
the  great  commissure,  for  the  reason  that  it  contains  white 
fibers  connecting  the  two  lateral  halves  of  the  cerebellum 
and  the  different  portons  of  the  cord  and  medulla  with  the 
parts  of  the  brain  above.  It  resembles  the  cord  in  having  its 
white  matter  situated  externally,  while  within  its  substance 
are  a  number  of  collections  of  gray  matter.  The  longitudi- 
nal fibers  are  continuations  upward  of  fibers  from  the  oli- 
vary bodies  and  the  anterior  pyramids  of  the  medulla  and 
also  of  parts  of  the  posterior  and  lateral  columns  of  the  cord. 
They  pass  through  the  crura  cerebri  to  the  brain. 

Functions. — The  anatomical  structure  and  situation  of  the 
pons  at  once  suggest  that  its  function  is  to  transmit  motor 
impulses  from  and  sensory  impressions  to  the  cerebrum. 

The  gray  centers,  however,  indicate  a  further  function  of 
this  organ.  It  is  found  that  the  removal  of  all  parts  of  the 
encephalon  above  the  pons  does  not  deprive  an  animal  of 
voluntary  motion  and  general  sensibility.  It  will  be  seen 


256  THE  NERVOUS  SYSTEM 

later  that  the  integrity  of  the  cerebrum  is  essential  to  any 
intellectual  operation,  and  manifestly,  under  the  conditions 
mentioned,  there  can  be  no  voluntary  motion  which  indi- 
cates any  degree  of  intelligence;  but  the  fact  remains  that 
the  animal  can  perform  movements  which  are  different  from 
the  reflex  movements  depending  on  the  presence  of  the  cord 
when  all  other  parts  of  the  cerebro-spinal  axis  have  been 
removed.  The  pons  is  apparently  "an  organ  capable  of 
originating  impulses  giving  rise  to  voluntary  movements, 
when  the  cerebrum,  corpora  striata  and  optic  thalami  have 
been  removed,  and  it  probably  regulates  the  automatic  vol- 
untary movements  of  station  and  progression."  (Flint.) 

Nor  can  it  be  doubted  that  an  animal  thus  mutilated  feels 
pain.  It  is  probable  that  the  sensory  impression  is  received 
by  some  of  the  gray  centers  in  the  pons  itself,  but  not  being 
conveyed  to  the  cerebrum,  is  not  remembered. 

The  Crura  Cerebri,  Corpora  Striata,  Optic  Thalami,  Inter- 
nal Capsule  and  Corpora  Quadrigemina. 

It  will  be  well  before  discussing  the  cerebrum  to  consider 
briefly  other  collections  of  gray  and  white  matter  in  the 
neighborhood  of  the  upper  part  of  the  pons. 

The  crura  cerebri,  passing  upward  from  the  anterior  part 
of  the  pons,  diverge  to  run  apparently  underneath  the  cor- 
pora striata  and  optic  thalami  iri  the  direction  of  the  cere- 
bral hemispheres.  They  are  about  %  mcn  l°ng  and  slightly 
broader  above  than  below.  The  main  bulk  of  each  crus  con- 
sists of  white  fibers,  but  a  collection  of  gray  matter  (locus 
niger)  divides  the  band  into  a  lower  or  superficial  section, 
called  the  crusta,  and  an  upper  or  deep  section,  called  the 
.tegmentum.  There  is  also  some  gray  matter  in  the  tegmen- 
tum  proper.  The  fibers  of  the  tegmentum  are  supposed  to 
convey  afferent  impressions  chiefly,  and  end  for  the  most 
part  in  the  optic  thalamus,  though  some  are  continued  to  the 
cerebrum  through  the  internal  capsule.  The  fibers  of  the 


THE  CRURA  CEREBRI 


257 


crusta  are  supposed  to  convey  efferent  impulses,  and  pass 
to  the  corpus  striatum  and  the  cerebrum. 

It  is  evident  that  the  function  of  the  cfura  is  mainly  to 


Nuclear  loitKomli 

Clauitnln* 


FIG.  80. — Human  brain,  with  the  hemispheres,  removed  by  a 
horizontal  incision  on  the  right  side. 

4,  trochlear;  8,  acoustic  nerve;  6,  origin  of  the  abducens;  F,  A,  L,  position  of 
the  pyramidal  (motor)  fibers  for  the  face,  arm  and  leg;  S,  sensory  fibers.  (Lan- 
dois. ) 

conduct  messages  to  and  from  the  parts  above.  It  is  said 
that  the  locus  niger  is  concerned  in  coordination  of  the  move- 
ments of  the  eye-ball  and  iris. 

17 


258  THE  NERVOUS  SYSTEM 

The  Corpora  Striata,  Optic  Thalami  and  Internal  Capsule 
are  closely  related  and  are  best  considered  together. 

Each  corpus  striatum  is  pear-shaped  with  its  large  end 
forward  and  near  the  median  line ;  the  posterior  small  ex- 
tremities are  divergent  from  each  other  and  embrace  the  two 
optic  thalami.  Externally  they  are  white;  internally  white 
and  gray  elements  are  mixed.  Each  is  separated  by  the  an- 
terior limb  of  the  internal  capsule  into  two  divisions,  exter- 
nal and  internal,  known  respectively  as  the  lenticular  and 
caudate  nuclei.  (See  Fig.  80.) 

The  optic  thalami,  one  on  either  side,  have  an  oval  shape 
and  rest  upon  the  crura  cerebri  between  the  posterior  ex- 
tremities of  the  two  corpora  striata.  Most  of  their  external 
surface  is  white ;  internally  each  possesses  six  gray  nuclei. 

Separating  the  two  nuclei  of  the  corpus  striatum  anteri- 
orly, and  the  lenticular  nucleus  from  the  optic  thalamus  pos- 
teriorly, is  a  band  of  white  fibers  known  as  the  internal  cap- 
sule. The  part  between  the  two  nuclei  is  the  interior  limb ; 
that  between  the  lenticular  nucleus  and  the  optic  thalamus  is 
the  posterior  limb.  These  limbs,  joining  at  an  obtuse  angle, 
constitute  a  bend  in  the  internal  capsule  which  is  called  the 
genu,  or  knee.  The  fibers  of  the  capsule  pass  to  the  frontal, 
parietal  and  occipital  lobes  of  the  cortex,  and  in  their  course 
to  these  parts  they  diverge  to  form  the  corona  radiata. 

External  to  the  lenticular  nucleus  is  a  band  of  white  fibers 
known  as  the  external  capsule.  In  it  is  a  longitudinal  mass 
of  gray  matter,  the  claustrum.  Fig.  76  shows  the  relations 
of  these  parts. 

Functions. — The  exact  function  of  the  corpora  striata  is 
a  matter  of  some  doubt.  They  have  been  considered  the 
great  motor  ganglia  of  the  base  of  the  brain ;  but,  although 
lesions  here  are  followed  by  paralysis  on  the  opposite  side  of 
the  body,  it  is  held  that  this  phenomenon  is  due  to  the  prox- 
imity of  the  internal  capsule.  The  further  fact  that  irrita- 
tion of  this  organ  is  followed  by  muscular  contraction  does 
not  prove  that  it  ordinarily  generates  motor  force,  for  many 


THE  CORPORA  QUADRIGEMINA  259 

of  the  fibers  from  the  motor  cortical  zone  pass  to  or  through 
the  corpus  striatum.  This  may  be  only  a  relay  station,  and 
the  corpus  may  be  quite  subsidiary.  It  undoubtedly,  how- 
ever, is  connected  with  motion  in  some  way. 

The  precise  function  of  the  optic  thalami  is  equally  ob- 
scure. The  relation  of  these  organs  to  the  tegmenta  would 
suggest  that  they  have  something  to  do  with  the  sensory 
fibers  on  their  way  to  the  cortex.  It  cannot  be  denied  that 
they  are  concerned  in  sensation,  since  their  removal  is  fol- 
lowed by  crossed  anesthesia.  They  may  likewise  be  relay 
stations.  Each  sends  fibers  to  the  cerebellum  and  contains 
one  of  the  nuclei  of  origin  of  the  optic  nerve. 

Regarding  the  function  of  the  internal  capsule  it  may  be 
said  that  its  fibers  are  in  main  part  prolongations  from  the 
crusta  and  from  the  gray  matter  of  the  corpora  striata; 
fibers  also  pass  upward  through  it  from  the  tegmentum  and 
the  optic  thalamus.  As  a  matter  of  fact,  most  of  the  fibers 
of  the  crura  go  directly  into  the  corpora  striata  (motor)  and 
the  optic  thalami  (sensory),  but  s*ome  pass  directly  upward 
through  the  capsule.  It  is  to  be  noted,  however,  that  the 
capsule  does  not  consist  of  these  last  named  fibers  alone,  but 
of  fibers  from  the  corpora  striata  and  optic  thalami  as  well. 
Observations  show  that  pathological  lesions  affecting  the 
anterior  two-thirds  of  the  posterior  division  of  the  internal 
capsule  are  followed  by  paralysis  of  motion;  that  lesions 
affecting  only  the  posterior  one-third  of  the  posterior  divi- 
sion are  followed  by  anesthesia;  and  that  lesions  affecting 
the  entire  posterior  limb  are  followed  by  both  paralysis  and 
anesthesia — these  phenomena  always  manifesting  themselves 
on  the  side  opposite  the  lesion  only.  This  leads  to  a  definite 
conclusion ;  viz.,  that  efferent  fibers  occupy  the  anterior  two- 
thirds  and  afferent  fibers  the  posterior  one-third  of  the  pos- 
terior limb  of  the  capsule. 

Nothing  conclusive  can  be  said  about  the  function  of  the, 
external  capsule  or  of  the  claustrum. 

The  Corpora  Quadrigemina,  two  on  each  side,  are  promi- 


260  THE  NERVOUS  SYSTEM 

nences  on  the  dorsal  surface  of  the  pons  and  crura  above  the 
aqueduct  of  Sylvius.  They  contain  white  and  gray  matter. 
The  posterior  tubercles  are  connected  with  the  eighth  nerve, 
the  sensory  tract,  the  temporal  region  of  the  brain,  and  the 
lateral  corpora  geniculata.  The  anterior  tubercles  are  con- 
nected with  the  optic  nerve,  with  the  occipital  region,  and 
with  the  median  corpora  geniculata. 

The  function  of  the  anterior  of  these  bodies  is  mainly 
connected  with  the  eye;  the  posterior  are  associated  with 
the  sense  of  hearing. 

The  Cerebrum, 

The  great  size  of  the  cerebral  hemispheres  in  man  ob- 
scures the  fact  that  the  different  parts  of  the  brain  are  dis- 
posed in  a  linear  series ;  these,  from  before  backward,  are, 
the  olfactory  lobes,  cerebral  hemispheres,  optic  thalami, 
corpora  quadrigemina,  cerebellum,  medulla  oblongata.  This 
arrangement  exists  in  theliuman  fetus,  and  persists  through- 
out life  in  some  of  the  lower  animals. 

Anatomy. — The  substance  of  each  hemisphere  is  divided 
by  fissures  into  five  lobes — (a)  frontal,  (b)  parietal,  (c) 
occipital,  (d)  temporo-sphenoidal  and  (e)  central.  The 
main  fissures  are  four  in  number — (i)  The  fissures  of  Syl- 
vius running  from  the  front  and  under  part  of  the  brain 
backward,  outward  and  upward;  (2)  the  fissures  of  Rolando 
running  from  the  median  line  near  the  center  of  the  longi- 
tudinal fissure  forward,  outward  and  downward;  (3)  the 
parie to  -occipital  fissure,  little  of  which  is  evident  upon  the 
surface  of  the  brain,  but  which  appears  on  longitudinal  sec- 
tion separating  the  occipital  and  parietal  lobes;  (4)  the 
calloso-marginal  fissure,  also  evident  only  on  the  internal 
aspect  of  the  hemisphere,  parallel  with  and  above  the  cor- 
pus callosum.  (Figs.  81,  82.) 

(a)  The  frontal  lobe  is  bounded  internally  by  the  longitu- 
dinal fissure,  posteriorly  by  the  fissure  of  Rolando  and  be- 


THE   CEREBRUM 


26l 


low  by  the  fissure  of  Sylvius.  On  its  surface  are  seen  three 
convolutions,  approximately  parallel,  called  the  superior, 
middle  and  inferior  frontal  convolution,  and  occupying  po- 
sitions which  their  names  indicate.  In  addition  the  posterior 


-cm 


FIG.  81.— Left  side  of  the. human  brain  (diagrammatic). 

F,  frontal;  P,  parietal;  O,  occipital;  T,  tempero-sphenoidal  lobe;  S,  fissure  of 
Sylvius;  S' ,  horizontal;  S",  ascending  ramus  of  S;  c,  sulcus  centralis,  or  fissure 
of  Rolando;  A,  ascending  frontal,  and  B,  ascending  parietal  convolution;  F\, 
superior,  FZ,  middle,  and  FS,  inferior  frontal  convolutions;  fi,  superior,  and  fz, 
inferior  frontal  fissures;  fz,  sulcus  precentralis;  P,  superior  parietal  lobule;  PZ, 
inferior  parietal  lobule,  consisting  of  PZ,  supra-marginal  gyrus,  and  PZ' ,  angular 
gyrus;  ip,  sulcus  interparietalis;  cm,  termination  of  callpso-marginal  fissure; 
O,  first,  Oz,  second,  Oz,  third  occipital  convolutions :po,  parietal-occipital  fissure; 
o.  transverse  occipital  fissure;  02,  inferior  longitudinal  occipital  fissure;  T\,  first, 
TZ,  second,  T%,  third  temporo-sphenoidal  convolutions;  t\,  first,  tz,  second  tem- 
pero-sphenoidal fissures.  (Landois.) 


262 


THE  NERVOUS  SYSTEM 


portion  of  this  lobe  is  occupied  by  the  ascending  frontal,  or 
the  anterior  central  convolution,  lying  just  in  front  of  the 
Rolandic  fissure. 

(b)  The  parietal  lobe  is  bounded  anteriorly  by  the  fissure 


FIG.  82. — Median  aspect  of  the  right  hemisphere. 

CC,  corpus  callosum  divided  longitudinally;  Gf,  gyrus  fornicatus;  H,  gyrus 
hippocampi;  h,  sulcus  hippocampi;  U,  uncinate  gyrus;  cm,  calloso-marginal  fis- 
sure; F,  first  frontal  convolution;  c,  terminal  portion  of  fissure  of  Rolando;  A, 
ascending  frontal;  B,  ascending  parietal  convolution  and  paracentral  lobule;  P\' , 
parecuneus  or  quadrate  lobule;  Os,  cuneus;  Po,  parieto-occipital  fissure;  o', 
transverse  occipital  fissure;  oc,  calcarine  fissure;  oc' ,  superior,  oc" ,  inferior 
ramus  of  the  same;  G' ,  gyrus  descendens;  T±,  gyrus  occipito-temporalis  lateralis 
(lobulus  fusiformis) ;  T§,  gyrus  occipito-temporalis  medialis  (lobulus  lingualis). 
(Landois.) 

of  Rolando,  internally  by  the  longitudinal  fissure,  posteriorly 
by  the  parieto-occipital  fissure  and  below  by  the  fissure  of 
Sylvius.  Just  behind  the  fissure  of  Rolando  is  the  ascending 
parietal,  or  posterior  central  convolution,  above,  this  is  con- 
tinuous with  the  upper  parietal  convolution,  below  which  is 
the  inferior  parietal  lobule  separated  from  the  preceding  by 
the  intra-parietal  sulcus.  This  inferior  parietal  lobule  winds 
around  the  posterior  part  of  the  fissure  of  Sylvius,  and  is 


THE  CEREBRUM  263 

divided  into  the  supra-marginal  convolution,  embracing  the 
short  arm  of  this  fissure,  and  the  angular  convolution  con- 
necting below  with  the  temporal  lobe. 

(c)  The  occipital  lobe  is  situated  posteriorly  below  the 
parieto-occipital  fissure  and  external  to  the  median  fissure. 
It  presents  three  convolutions,  the  superior,  middle  and  in- 
ferior. 

(d)  The  temporo-sphenoidal  lobe  is  below  the  fissure  of 
Sylvius  in  front  of  the  occipital  lobe.     It  likewise  presents 
superior,  middle  and  inferior  convolutions. 

(e)  The  central  lobe,  or  island  of  Reil,  presents  the  gyrus 
fornicatus,  a  convolution  curving  around  the  corpus  cal- 
losum ;  the  marginal  convolutions  beyond  the  calloso-mar- 
ginal  fissure  from  the  preceding  and  between  it  and  the  edge 
of  the  longitudinal  fissure;  the  continuation  of  the  parieto- 
occipital  fissure  running  downward  and  forward  to  meet  the 
calcarine  fissure,  between  which  is  the  cuneus;  the  internal 
aspect  of  the  temporal  lobe,  the  uncinate  gyrus. 

Structure. — The  cerebral  hemispheres  are  composed  of 
white  and  gray  matter,  but  here  the  gray  matter  is  situated 
externally.  To  increase  its  amount,  with  economy  of  space, 
the  gray  matter  is  thrown  into  many  convolutions,  to  some 
of  which  reference  has  been  made.  The  sulci  separating 
these  convolutions  have  a  depth  in  the  average  human  brain 
of  about  one  inch.  The  thickness  of  the  gray  matter  of  the 
cortex  varies  from  1/i2  to  %  in.,  being  thinnest  in  the  occipital 
and  thickest  in  the  front  parietal  region. 

The  cells  found  in  the  superficial  and  deep  portions  of  the 
gray  matter  are  not  uniform  in  size  or  shape.  In  a  general 
way  it  may  be  said  that  they  increase  in  size  as  the  surface 
is  left,  but  in  addition  to  the  comparatively  large  cells  in  the 
deep  parts  there  are  also  numbers  of  small  ones.  Passing  in 
the  same  direction  there  are  found  in  succession  small  pyra- 
midal, larger  pyramidal,  and  irregular  branching  cells. 

Fibers  from  the  Cerebrum. — Fibers  pass  from  each  cere- 
bral hemisphere  to  (a)  the  spinal  cord,  (b)  the  cerebellum, 


264  THE  NERVOUS  SYSTEM 

(c)    the   opposite   cerebral   hemisphere,   and    (d)    different 
parts  of  the  same  hemisphere. 

(a)  Fibers  converge  from  the  anterior  and  middle  (par- 
ticularly the  latter)  parts  of  the  cortex  to  pass  by  the  corona 


FIG.  83. — Scheme  of  the  projection  fibers  within  the  brain.     (Starr.) 

Lateral  view  of  the  internal  capsule;  A,  tract  from  the  frontal  gyri  to  the 
pons  nuclei,  and  so  to  the  cerebellum;  B,  motor  tract;  C,  sensory  tract  for  touch 
(separated  from  B  for  the  sake  of  clearness  in  the  scheme);  D,  visual  tract;  E, 
auditory  tract;  F,  G,  H,  superior,  middle,  and  inferior  cerebellar  peduncles;  /, 
fibers  between  the  auditory  nucleus  and  the  inferior  quadrigeminal  body;  K, 
motor  decussation  in  the  bulb;  At,  fourth  ventricle.  The  numerals  refer  to  the 
cranial  nerves.  The  sensory  radiations  are  seen  to  be  massed  toward  the 
occipital  end  of  the  hemisphere.  (Am.  Text-book.) 

radiata  to  the  corpora  striata,  from  which  fibers  are  con- 
tinued to  the  crusta,  pons,  pyramids  of  the  medulla  and 
pyramidal  tracts  of  the  cord;  most  of  these  pass  down 
through  the  internal  capsule  to  reach  the  corpora  striata. 
From  the  same  regions  also  some  fibers  pass  directly  through 
the  internal  capsule,  without  connection  with  the  corpora 
striata,  to  be  actually  continuous  themselves  with  fibers 
which,  following  the  same  course  downward,  are  found  in 
the  pyramidal  tracts  of  the  cord.  All  fibers  passing  from 
these  cortical  areas  mentioned  through  the  internal  capsule 


THE  CEREBRUM 


265 


FIG.  84. — Scheme  of  relationship  of  cells  and  fibers  of  brain  and  cord. 

(Kirkes.) 

Pyr,  cell  of  Rolandic  area;  Ax,  its  axis  cylinder  crossing  the  middle  line  AB, 
to  enter  one  of  the  pyramidal  tracts;  the  collateral  Call  goes  to  the  cortex  of 
the  opposite  hemisphere,  while  another,  str,  enters  the  corpus  striatum.  The  axis 
cylinder  arborizes  around  an  anterior  horn  cell,  whence  a  motor  fiber  goes  to  the 
muscle. 

The  axis  cylinder  from  the  spinal  ganglion  cell  is  represented  as  bifurcating 
and  sending  one  branch  to  the  periphery  and  one  to  the  cord;  the  latter  itself 
bifurcates,  the  lower  division  ending  as  shown  better  in  Fig.  77.  N.G.,  cell  in 
posterior  cornu  of  the  cord  or  posterior  column  of  the  bulb.  The  distance  of 
this  cell  from  the  point  of  entrance  of  the  axis  cylinder  into  the  cord  may  be 
great  or  small.  Note  the  collaterals  from  it  in  Fig.  77.  I. A.,  decussating  fiber 
ending  at  cell  in  optic  thalamus,  O.T.,  from  which  a  fiber  passes  to  the  cortex. 
A  collateral  is  shown  passing  from  the  ascending  sensory  fiber  to  a  cell  of 
Clarke's  column,  whence  a  fiber  passes  to  a  cell,  P,  of  the  cerebellum. 


266  THE  NERVOUS  SYSTEM 

occupy  the  anterior  two-thirds  of  the  posterior  division  of 
that  tract.  Furthermore,  fibers  from  the  posterior  cortical 
area  pass  through  the  posterior  one-third  of  the  posterior 
division  of  the  internal  capsule  to  the  optic  thalamus,  from 
which  fibers  pass  through  the  tegmentum  to  the  pons  and 
medulla  and  are  continuous  with  fibers  from  the  sensory 
tracts  of  the  cord.  The  decussation  of  all  these  fibers  has 
been  mentioned. 


FIG.  85. — Diagram  of  the  motor  areas  on  the  outer  surface  of  a 
monkey's  brain.     (Landois  after  Horsley  and  Schafer.) 

Fig.  84  taken  in  conjunction  with  Fig.  77  illustrates  the 
most  recent  ideas  of  the  motor  and  sensory  connections  be- 
tween brain  and  cord  and  the  motor  and  sensory  paths  in 
the  cord. 

(b)  Fibers  from  the  anterior  portion  of  the  frontal  lobe 
pass  through  the  anterior  limb  of  the  internal  capsule  and 
seem  to  end  in  the  gray  matter  of  the  pons  and  there  to  com- 
municate with  the  cerebellum  through  the  middle  peduncles. 
Fibers  also  pass  from  the  temporo-sphenoidal  lobes  and 
from  the  caudate  nuclei  of  the  corpora  striata  to  the  cere- 
bellum on  the  opposite  side.  The  connection  is  crossed  in  all 
these  cases. 

(r)  Transverse  fibers  in  the  corpus  callosum  connect  all 
parts  of  the  two  lateral  hemispheres.  Besides  these  com- 


THE  CEREBRUM 


FIG.  86. — Side  view  of  the  brain  of  man,  with  the  areas  of  the  cerebral 
convolutions  according  to  Ferrier.     (Brubaker.) 


The  figures  are  constructed  by  marking  on  the  brain  of  man,  in  their  respec- 
tive situations,  the  areas  of  the  brain  of  the  monkey  as  determined  by  experi- 
ment, and  the  description  of  the  effects  of  stimulating  the  various  areas  refers 
to  the  brain  of  the  monkey. 

i,  advance  of  the  opposite  hind  limb,  as  in  walking;  2,  3,  4,  complex  move- 
ments of  the  opposite  leg  and  arm,  and  of  the  trunk,  as  in  swimming;  a,  b,  c,  d, 
individual  and  combined  movements  of  the  fingers  and  wrist  of  the  opposite 
hand.  Prehensile  movements.  5,  extension  forward  of  the  opposite  arm  and 
hand;  6,  supination  and  flexion  of  the  opposite  forearm;  7,  retraction  and  ele- 
vation of  the  opposite  angle  of  the  mouth  by  means  of  the  zygomatic  muscle; 
8,  elevation  of  the  alae  nasi  and  upper  lip,  with  depression  of  the  lower  lip  on 
the  opposite  side;  9,  10,  opening  of  the  mouth,  with  (9)  protrusion  and  (10) 
retraction  of  the  tongue;  region  of  aphasia,  bilateral  action;  n,  retraction  of 
the  opposite  angle  of  the  mouth,  the  head  turned  slightly  to  one  side;  12,  the 
eyes  open  widely,  the  pupils  dilate,  and  the  head  and  eyes  turn  toward  the  oppo- 
site side;  13,  13',  the  eyes  move  toward  the  opposite  side,  with  an  upward  (13) 
or  downward  (13')  deviation;  the  pupils  are  generally  contracted;  14,  pricking 
of  the  opposite  ear,  the  head  and  eyes  turn  to  the  opposite  side,  and  the  pupils 
dilate  widely. 


268  THE  NERVOUS  SYSTEM 

missural  fibers  there  are  those  of  the  anterior  and  posterior 
white  commissures.  Fibers  in  the  anterior  connect  the  tem- 
poro-sphenoidal  lobes  and  probably  the  corpora  striata  with 
each  other ;  fibers  in  the  posterior  connect  the  temporo-sphe- 
noidal  lobes  with  the  optic  thalami  of  the  opposite  side. 

(d)  The  arcuate  fibers  connect  different  convolutions  of 
the  same  lobe  and  the  convolutions  of  different  lobes  with 
each  other.  Some  of  these  are  the  jornix,  in  the  corpus 
callosum}  and  in  the  other  parts,  as  well  as  running  along 
the  concave  surface  of  the  cortex. 

Cerebral  Localization. — There  are  certain  cortical  areas 
which  have  certain  fixed  functions.  There  are  certainly  such 
areas  for  motion  and  for  the  reception  of  impressions  con- 
veyed by  the  nerves  of  special  sense;  areas  for  the  reception 
of  impressions  conveyed  by  the  nerves  of  general  sensation 
have  not  been  definitely  determined. 

Motor  Centers. — Electrical  stimulation  of  the  convex  sur- 
face of  the  cerebrum  shows  that  the  anterior  part  is  motor 
and  the  posterior  part  non-motor;  that  stimulation  of  the 
motor  portion  produces  muscular  contractions  on  the  oppo- 
site side  of  the  body,  that  stimulation  in  the  same  spot  is  al- 
ways followed  by  the  same  contractions;  and  that  when  the 
current  is  quite  weak  the  contractions  are  limited  to  distinct 
muscles  or  sets  of  muscles.  It  may  be  further  said  that  while 
the  experiments  establishing  these  facts  have  been  largely 
limited  to  inferior  animals,  the  deductions  have  been  made 
applicable  to  man  by  pathological  observations  and  by  the 
fact  that  in  different  animals  stimulation  of  anatomically 
corresponding  parts  is  followed  by  corresponding  results. 
Destruction  of  motor  areas  is  followed  by  descending  sec- 
ondary degeneration  of  fibers  through  the  corona  radiata, 
internal  capsule,  crura  cerebri  (crusta),  anterior  pyramids 
of  the  medulla  and  the  pyramidal  tracts  of  the  cord;  the 
resulting  paralysis  is  on  the  side  opposite  the  lesion. 

The  motor  cortical  zone,  so  far  as  can  now  be  said,  cor- 
responds to  the  ascending  frontal  and  parietal  convolutions 


THE  CEREBRUM  269 

on  either  side  of  the  fissure  of  Rolando,  to  the  paracentral 
lobule,  and  possibly  to  a  small  area  in  front  of  the  ascending 
frontal  convolution.  From  above  downward,  on  either  side 
o.f  the  Rolandic  fissure  are  areas  presiding  over  the  move- 
ments of  the  leg,  arm  and  face. 

More  specific  information  as  regards  areas  controlling 
various  movements  may  be  obtained  by  reference  to  Fig.  86. 

Various  kinds  of  monoplegia  (crossed)  are  caused  by 
lesions,  as  hemorrhage,  in  localized  parts  of  the  motor  area ; 
there  may  be  facial,  brachial,  crural,  bracho-facial  monople- 
gia, etc.  There  can  be  no  doubt  that  from  the  motor  cortical 
zone  pass  the  fibers  which  constitute  the  pyramidal  tracts  of 
the  cord. 

Sensory  Centers. — Centers  for  the  reception  of  impres- 
sions giving  rise  to  general  sensation  may  exist.  Fibers  from 
the  temporo-sphenoidal  and  occipital  lobes  pass  through  the 
posterior  third  of  the  posterior  division  of  the  internal  cap- 
sule, and  it  may,  therefore,  be  assumed  that  these  parts  of 
the  cerebrum  are  connected  with  general  sensation. 

Special  Centers. — Besides  these  areas  for  motion  and  gen- 
eral sensation,  special  centers  certainly  exist. 

The  Optic  Center  is  in  the  occipital  lobe,  probably  in  the 
cuneus.  Removal  of  the  right  occipital  lobe  is  followed  by 
left  hemiopia  and  vice  versa;  removal  of  both  causes  total 
blindness. 

The  Olfactory  Center  is  probably  on  the  inner  surface  of 
the  anterior  extremity  of  the  uncinate  gyrus  (inner  extrem- 
ity of  the  temporal  lobe). 

The  Gustatory  Center  is  supposed  to  be  in  the  temporal 
lobe  very  near  the  preceding. 

The  Auditory  Center  is  located  in  the  superior  and  middle 
convolutions  of  the  temporo-sphenoidal  lobe. 

The  Center  for  Cutaneous  Sensations  cannot  be  strictly 
limited,  though  it  is  said  to  correspond  with  the  motor  area. 

The  Center  for  Muscular  Sensations  is  thought  to  be  in 
the  lower  parietal  region. 


2/O  THE  NERVOUS  SYSTEM 

The  Speech  Center. — One  may  not  be  able  to  speak  be- 
cause he  cannot  control  the  muscles  usually  involved  in  such 
an  act,  or  because  he  has  no  comprehension  of  the  meaning 
of  words,  or  because  he  is  incapable  of  forming  the  idea 
which  links  the  reception  of  the  impression  and  the  muscu- 
lar act.  Aphasia  is  the  term  generally  applied  to  inability  to 
express  one's  self  by  language.  It  is  to  be  distinguished, 
however,  from  aphonia,  which  is  simply  a  loss  of  voice. 
Ataxic  aphasia  is  an  inability  to  express  ideas  only  by  reason 
of  muscular  incoordination ;  a  person  so  affected  may  use 
words,  but  he  cannot  tell  what  sounds  he  is  going  to  utter ; 
his  ability  to  receive  ideas  is  unimpaired,  and  he  can  express 
his  own  ideas  in  writing.  When  there  is  inability  to  express 
ideas  in  writing,  because  of  muscular  incoordination,  a  con- 
dition of  agraphic  aphasia  is  said  to  exist.  There  are  also 
cases  in  which  a  person  cannot  comprehend  ideas  expressed 
in  language  and  cannot  express  himself  by  either  speaking 
or  writing;  this  is  known  as  amnesic  aphasia.  It  is  not  im- 
possible that  in  some  instances  ideas  may  be  received  and 
there  still  be  an  inability  to  express  one's  self  in  any  way. 
It  is  noted  that  when  the  hemiplegia  accompanying  the 
aphasia  is  marked  the  form  is  usually  ataxic ;  when  there  is 
no  hemiplegia  the  aphasia  is  usually  amnesic. 

The  part  of  the  brain  presiding  over  speech  is  in  the  left 
third  frontal  convolution  near  the  island  of  Reil.  In  left- 
handed  persons  its  usual  situation  is  almost  certainly  at  a 
corresponding  point  on  the  right  side.  Why  the  center  is 
unilateral  has  -not  been  explained.  It  may  be  that  it  was 
originally  bilateral,  and  the  growth  of  the  right  has  been 
stopped  by  the  superior  development  of  the  left  side  of  the 
brain.  It  is  at  least  noticed  that  the  right  instead  of  the  left 
side  of  the  brain  is  heavier  in  left-handed  persons.  Fibers 
from  this  center  (Broca's  convolution)  pass  through  the  an- 
terior part  of  the  posterior  division  of  the  internal  capsule 
to  reach  the  left  crus,  leaving  which  they  enter  the  pons  to 
decussate  and  go  to  the  right  side  of  the  medulla. 


THE  CEREBRUM  2JI 

Functions  of  the  Cerebrum. — The  superior  development 
of  the  intellect  in  man  is  the  most  predominant  characteristic 
distinguishing  him  from  the  lower  animals.  That  many  such 
animals  are  possessed  of  a  certain  degree  of  intelligence  is 
not  usually  denied ;  and  the  nature  of  their  mental  oper- 
tions,  though  they  are  insignificant  as  compared  with  man's, 
may  be  admitted  as  identical  with  his.  The  most  striking 
difference  in  the  nervous  system  of  man  as  compared  with 
that  of  inferior  animals  is  the  large  size  of  the  cerebrum  in 
the  former.  This  is  not  surprising  when  it  is  admitted  that 
in  the  substance  of  this  part  of  the  encephalon  is  the  seat  of 
those  faculties  which  manifest  themselves  in  mental  opera- 
tions. 

The  seat  of  the  changes,  if  they  be  changes,  which  result 
in  mental  operations  is  supposed  to  be  in  the  frontal  lobes; 
these  are  insensible  and  inexcitable,  but  severe  injury  to 
them,  as  by  hemorrhage,  is  followed  by  a  cessation  of  mental 
activity;  congenital  defects  also  cause  a  corresponding  de- 
crease in  the  mental  caliber. 

From  what  has  been  said  it  is  evident  that  the  cerebral 
hemispheres  are  capable  of  generating  motor  impulses  and 
receiving  impressions  general  and  special;  but  predominat- 
ing in  importance  over  these  functions  is  the  fact  that  the 
gray  substance  of  the  cerebrum  is  essential  to  the  exercise 
of  the  intellect — even  to  the  existence  of  that  indefinite  some- 
thing called  the  mind. 

It  is  by  the  cerebrum  that  we  perceive  and  retain  impres- 
sions, that  we  understand,  imagine,  reflect,  reason  and  judge, 
and  thus  concoct  and  issue  the  mandates  of  our  will.  It  is 
the  link  which  connects  our  impressions  and  our  purposeful 
actions. 

In  animals  upon  which  experiments  have  been  made  it  is 
found  that  life  may  persist  for  a  time  after  the  removal  of 
the  hemispheres,  and  that,  outside  of  the  cessation  of  men- 
tal activity,  the  results  are  not  so  marked  as  one  would  on 
first  thought  suppose.  Stupor  and  absence  of  the  ordinary 


2/2  THE  NERVOUS  SYSTEM 

instinctive  acts  (as  corresponding  in  a  way  with  ac.ts  of  the 
will  in  man)  are  noted,  but  voluntary  motion  and  general 
sensibility  are  not  destroyed,  and  may  be  but  little  inter- 
fered with.  Of  course  there  is  no  voluntary  motion  in  the 
sense  of  carrying  out  the  behests  of  the  will,  for  the  organ 
of  the  will  is  destroyed ;  nor  is  there  any  record  of  painful 
impressions,  for  the  organ  of  memory  is  absent.  But  the 
animal  can  perform  various  consecutive  and  coordinate 
movements,  such  as  walking,  swimming,  etc.  For  example, 
a  pigeon  thus  mutilated  will  fly  when  thrown  into  the  air. 
This  does  not  argue  any  mental  operation.  A  person  does 
not  ordinarily  apply  his  mind  to  the  act  of  walking  or  stand- 
ing; his  mental  faculties  may  be  as  completely  engaged  with 
the  deepest  thoughts  of  psychology,  literature,  medicine  or 
other  subjects  while  walking  as  at  any  other  time.  True,  he 
probably  started  with  some  fixed  purpose  to  go  in  some  par- 
ticular direction  to  some  definite  place,  but  the  act  of  pro- 
gression does  not  per  se  require  fixed  attention  on  his  part. 
So  in  the  case  of  the  pigeon ;  it  does  not  make  up  its  mind 
to  fly  at  all;  and  it  will  not  fly  without  being  thrown  into 
the  air,  or  the  application  of  some  other  similar  stimulus ; 
nor  does  it  fly  in  any  particular  direction,  or  to  any  par- 
ticular place.  It  is  reduced  to  the  condition  of  a  "mechan- 
ism without  spontaneity."  It  can  perform  voluntary  move- 
ments but  cannot  originate  them  without  external  interven- 
tion. 

Animals  which  have  been  subjected  to  the  operation  men- 
tioned undoubtedly  feel  pain.  They  move  away  or  cry  out 
on  being  burned,  for  example.  The  coordination  of  their 
movements  and  the  cries  contrast  with  the  phenomena  (re- 
flex) following  such  stimulation  when  only  the  cord  is  left. 
It  was  noted  above  that  the  impressions  in  these  cases  are 
probably  received  by  the  gray  matter  of  the  pons  and  not 
recorded. 

The  special  senses  of  sight  and  hearing  remain  after  the 
removal  of  the  cerebrum.  The  same  is  probably  true  of 
taste  and  smell. 


THE  CEREBRUM  273 

It  would  seem  that  the  cerebrum  is  a  kind  of  storehouse 
in  which  are  kept  all  the  materials  necessary  for  the  per- 
formance of  all  kinds  of  pro-determined  acts,  whether  they 
manifest  themselves  in  speech,  or  thought,  or  muscular 
action.  What  excites  these  materials  to  activity — i.  e.,  what 
excites  a  voluntary  act — is  not  clear.  We  know  certain 
things  will  usually  excite  a  certain  train  of  thought,  or  cause 
us  to  will  to  do  or  say  certain  things.  Such  phenomena*  are 
akin  to,  if  not  identical  with,  reflex  action.  These  manifes- 
tations of  our  voluntary  power  are  due  to  impressions  con- 
veyed by  afferent  fibers  to  the  cortex ;  indeed  it  may  be  that 
every  afferent  fiber  in  the  system  exerts  an  influence  thus 
indirectly  upon  the  organ  of  the  will,  "and  the  impressions 
conveyed  by  them  are  reflected  in  one's  character  and  life. 
But  it  cannot  be  said  that  all  voluntary  activity  is  thus  of  a 
reflected  nature;  there  is  some  cause  other  than  the  recep- 
tion of  afferent  impressions  which  sets  the  will  in  operation. 

Connection  Between  the  Brain  and  Intelligence. — It  is 
claimed  that  a  single  hemisphere  is  capable  of  performing  all 
the  ordinary  intellectual  acts  as  well  as  both;  and  atrophy, 
or  destruction  otherwise,  of  one  hemisphere  has  frequently 
been  noticed  to  entail  no  mental  defect.  But  whether  the 
mind  under  such  conditions  would  be  equal  to  the  highest 
intellectual  attainments  is  doubtful.  It  would  seem  that  in 
health  the  brain  unites  the  impressions  received  by  the  two 
sides  (as  e.  g.,  through  the  optic  nerves),  and  the  resulting 
idea  is  a  single  one;  that  is  to  say  a  person  does  not  have 
two  opposing  ideas  about  the  same  thing  the  same  time ;  the 
two  hemispheres  seem  to  agree. 

In  a  general  way,  it  may  be  stated  that  the  degree  of  in- 
telligence corresponds  to  the  weight  of  the  brain,  though 
to  this  rule  there  are  many  exceptions.  It  may  be  more  prop- 
erly said  that  the  development  of  the  intellectual  faculties  is 
greater  as  the  area  of  gray  matter  is  increased  by  the  convo- 
lutions of  the  cortex.  Idiots'  brains  are  usually,  though  not 
by  any  means  invariably,  much  below  the  average  weight. 


274  THE  NERVOUS  SYSTEM 

A  difference  in  intellectual  vigor  may  be  present  in  per- 
sons whose  brains  have  the  same  weight  and  even  the  same 
amount  of  gray  matter.  A  difference  in  the  quality  of  the 
gray  substance  may  in  such  cases  account  for  the  varying 
results.  It  is  a  matter  of  common  observation  that  mental 
exercise  increases  mental  vigor  and  capacity,  just  as  muscu- 
lar exercise  develops  muscular  strength.  It  is  difficult  to 
reach  a  conclusion  as  to  whether  there  is  an  increase  in  the 
amount  of  gray  substance  or  whether  that  already  present  is 
endowed  with  additional  power. 

The  Cerebellum. 

Anatomy. — The  cerebellum,  or  little  brain  (see  Fig.  79), 
is  situated  beneath  the  occipital  lobes  of  the  cerebrum, 
weighs  some  5*4  ounces  in  the  male  to  4^  ounces  in  the  fe- 
male, and  consists  of  a  central  and  two  lateral  lobes.  It  is 
composed  of  white  and  gray  matter,  the  latter  being,  with 
the  exception  of  the  corpora  dentata  in  the  lateral  lobes,  sit- 
uated externally.  The  convolutions  on  its  surface  are  much 
finer  than  are  those  on  the  cerebral  surface.  It  is  separated 
from  the  parts  above  by  the  tentorium  cerebelli,  a  pro- 
cess of  the  dura  mater. 

Fibers. — The  fibers  passing  away  from  the  cerebellum  are 
collected  into  three  bundles  on  each  side,  known  as  the  su- 
perior, middle  and  inferior  peduncles.  The  superior  pe- 
duncle has  a  direction  forward  and  upward  to  reach  the  crus 
and  optic  thalamus ;  fibers  in  it  connect  the  cerebellum  with 
the  cerebrum.  Certain  of  these  decussate  underneath  the 
corpora  quadrigemina  with  corresponding  fibers  from  the 
opposite  side,  so  that  each  side  of  the  cerebellum  is  connected 
with  both  sides  of  the  cerebrum.  Attention  has  been  called 
to  fibers  passing  down  from  the  cerebrum  through  the  pons 
to  the  cerebellum.  Fibers  in  the  middle  peduncle  connect 
the  two  lateral  halves  of  the  cerebellum  through  the  pons. 
Fibers  in  the  inferior  peduncle  are  continuous  below  with 


THE  CEREBELLUM  275 

fibers  in  the  posterior  columns  of  the  cord  through  the  resti- 
form  bodies  of  the  medulla. 

Function. — The  only  characteristic  phenomenon  invariably 
following  removal  of  the  cerebellum  is  an  inability  to  coor- 
dinate the  voluntary  muscular  movements.  The  foot,  for 
example,  can  be  raised,  and  the  voluntary  muscular  act  con- 
cerned in  raising  it  may  be  as  vigorous  as  ever,  but  the  ani- 
mal cannot  so  govern  his  movements  as  to  know  where  he  put 
it  down.  Even  the  coordination  necessary  in  standing  is  lost, 
and  the  maintenance  of  the  equilibrium  is  very  difficult,  if  not 
impossible.  The  so-called  muscular  sense  is  abolished,  and, 
while  the  power  to  contract  the  muscles  remains,  the  animal 
cannot  contract  them  in  a  regular  or  coordinate  manner. 
When  it  is  remembered  that  wellnigh  every  voluntary  act 
requires  concerted  or  consecutive  muscular  movements  some 
idea  is  gotten  of  the  helpless  condition  sequent  upon  such 
a  lesion.  If  it  be  granted  that  there  is  a  center  presiding 
over  the  coordination  of  the  voluntary  muscles,  that  center 
is  in  the  cerebellum,  and  an  animal  deprived  of  this  organ  is 
as  powerless,  so  far  as  this  function  is  concerned,  as  a  per- 
son is  to  see  when  the  optic  centers  are  destroyed.  Its  action 
is  crossed. 

It  has  been  noted  already  that  lesions  of  the  posterior 
white  columns  of  the  cord  are  followed  by  disturbances  of 
coordination,  and  that  the  cerebellum  is  connected  with  these 
columns  through  the  inferior  peduncles  and  restiform  bodies. 
Fibers  in  these  columns  serve  only  as  anatomical  connec- 
tions by  which  the  coordinating  center  communicates  with 
the  muscles  whose  movements  it  is  to  regulate,  and  of  ne- 
cessity any  lesion  of  these  fibers  destroying  that  connection 
is  followed  by  the  loss  of  control  of  the  center  over  the  mus- 
cles. However,  in  degeneration  of  the  posterior  columns 
(locomotor  ataxia)  an  effort  at  coordination  can  be  made, 
so  that  progression  is  possible  by  the  aid  of  fixed  attention. 
It  is  possible  also  that  the  coordinating  messages  are  carried 
in  such  cases  by  the  motor  fibers,  though  in  an  unsatisfactory 
manner. 


276  THE   NERVOUS   SYSTEM 

It  has  been  supposed  that  the  cerebellum  is  in  some  way 
connected  with  the  generative  function,  and  this  much  is 
probably  true,  though  the  evidence  submitted  is  not  suffi- 
cient to  warrant  the  assumption  that  the  cerebellum  is  the 
seat  of  the  sexual  instinct. 

THE  CRANIAL  NERVES. 

The  cranial  nerves,  twelve  in  number  on  each  side,  take 
their  origin  from  some  part  of  the  encephalon,  pierce  the 
dura  mater  and  leave  the  skull  by  various  openings.  They 
have  been  numbered  from  before  backward  in  the  order  in 
which  they  pass  through  the  dura  mater.  Their  names, 
indicating  something  of  their  function,  and  corresponding  to 
their  numbers,  are  as  follows : 

I.  Olfactory. 
II.  Optic. 

III.  Motor  Oculi  Communis. 

IV.  Patheticus  (Trochlearis). 
V.  Trifacial  (Trigeminus). 

VI.  Abducens. 
VII.  Facial. 
VIII.  Auditory. 
IX.  Glosso-pharyngeal. 

X.  Pneumogastric  (Vagus). 
XI.  Spinal  Accessory. 
XII.  Hypoglossal. 

The  point  at  which  one  of  these  nerves  can  be  seen  to  issue 
from  the  brain  tissue  is  the  apparent  origin,  while  the  gray 
nucleus,  or  nuclei,  to  which  the  fibers  can  be  traced  in  the 
brain  substance  is  the  deep  origin. 

First  Nerve  (Olfactory). 

Origin. — This  is  a  nerve  of  special  sense.  Its  apparent 
origin  is  by  three  roots.  The  internal  root  issues  from  the 


THE  CRANIAL  NERVES  2/7 

gyrus  fornicat.us;  the  middle  from  the  under  surface  of  the 
frontal  lobe  anterior  to  the  anterior  perforated  space;  the 
external  from  the  temporo-sphenoidal  lobe.  These  three 
roots  unite  to  pass  forward  underneath  the  frontal  lobe  near 
the  longitudinal  fissure  as  the  olfactory  tract.  The  deep 
origin  is  unsettled. 

Course  and  Distribution. — Reaching  the  upper  surface  of 
the  cribriform  plate  of  the  ethmoid,  the  olfactory  tract  ex- 
pands into  the  olfactory  bulb,  from  the  under  surface  of 
which  are  given  off  the  special  nerve  fibers  of  the  sense  of 
smell.  They  are  about  twenty  in  number  and  pass  through 
the  foramina  in  the  cribriform  plate  to  be  distributed  to  the 
mucous  membrane  (Schneiderian)  of  the  nose  in  three  sets 
— an  inner  to  the  upper  third  of  the  septum,  a  middle  to  the 
roof  of  the  nares,  and  an  outer  to  the  superior  and  middle 
turbinated  bones  and  the  ethmoid  in  front  of  them.  The 
fibers  are  non-medullated. 

Function. — The  olfactory  nerves  are  insensible  and  inex- 
citable.  They  are  concerned  with  the  sense  of  smell  alone 
and  their  integrity  is  necessary  to  the  preservation  of  that 
sense.  They  convey  to  the  brain  impressions  which  are  rec- 
ognized as  odors  only.  Removal  of  the  olfactory  bulb  in  a 
dog  is  evidently  followed  by  a  loss  of  the  sense  so  charac- 
teristic of  the  animal.  Furthermore,  the  olfactory  bulbs  in 
lower  animals  are  shown  to  be  developed  in  proportion  to  the 
acuteness  of  the  sense  of  smell. 

Second  Nerve  (Optic). 

Origin. — This  is  the  nerve  of  sight.  Its  apparent  origin  is 
from  the  anterior  part  of  the  optic  cgmmissure.  The  optic 
commissure  occupies  the  optic  groove 'on  the  superior  sur- 
face of  the  sphenoid.  It  represents  the  union  of  the  two 
optic  tracts  each  of  which,  traced  backward,  is  found  to 
divide  into  two  bands ;  the  external  takes  its  origin  from  the 
external  geniculate  b.ody,  from  the  pulvinar  of  the  optic  thai- 


2/8  THE  NERVOUS  SYSTEM 

amus  and  from  the  superior  corpus  quadrigeminum ;  the  in- 
ternal comes  from  the  internal  geniculate  body.  These  two, 
uniting,  cross  the  crusta  obliquely  to  reach  the  optic  commis- 
sure, or  chiasm.  In  the  commissure  the  fibers  from  the  inner 
margin  of  each  optic  tract  pass  to  the  other  side  of  the  brain, 
and  may  be  called  commissural  fibers  between  the  internal 
geniculate  bodies.  Some  fibers  anteriorly  connect  the  two 
optic  nerves  with  each  other  and  are  not  properly  part  of 
the  chiasm,  but  connect  the  two  retinae.  The  outer  fibers  of 
each  tract  pass  to  the  nerve  of  the  same  side,  while  the 
central  fibers  decussate  in  the  commissure  with  similar  fibers 
from  the  other  tract  and  pass  thus-  to  the  optic1  nerve  of  the 
opposite  side.  The  deep  origin  is  indicated  above. 

Course  and  Distribution. — Each  optic  nerve  leaves  the 
front  of  the  optic  chiasm  to  pass  out  of  the  cranium  and 
enter  the  orbital  cavity  by  the  optic  foramen.  Having 
pierced  the  sclerotic  and  choroid  coats  of  the  ball  it  expands 
into  the  retina. 

Function. — The  optic  nerves  have  no  properties  other 
than  the  conveying  to  the  brain  of  the  special  impressions  of 
sight.  Stimulation  produces  neither  pain  nor  motion. 

Third  Nerve  (Motor  Oculi  Communis). 

Origin. — The  third  is  a  motor  nerve.  Its  apparent  origin 
is  from  the  inner  surface  of  the  crus  just  in  front  of  the 
pons  Varolii.  Its  deep  origin  is  in  a  nucleus  just  lateral  to 
the  median  line  beneath  the  aqueduct  of  Sylvius.  Here  de- 
cussation  with  fibers  from  the  opposite  side  occurs.  The 
fibers  pass  forward  from  this  place  through  the  locus  niger 
and  tegmentum  to  the  point  of  apparent  origin. 

Course  and  Distribution. — Having  traversed  .the  outer 
aspect  of  the  cavernous  sinus,  the  third  nerve  divides  into 
two  branches  which  leave  the  cranial  cavity  by  the  sphe- 
noidal  fissure  between  the  two  heads  of  the  external  muscle 
of  the  eye.  The  superior  division  is  distributed  to  the  su- 


THE  CRANIAL  NERVES  2/9 

perior  rectus  and  levator  palpebrae  superioris;  the  inferior 
separates  into  three  branches,  one  of  which  is  distributed  to 
the  inferior  rectus,  another  to  the  internal  rectus,  and  a 
third  to  the  inferior  oblique.  From  this  last  a  branch  is  given 
off  to  the  lenticular  ganglion  to  form  its  inferior  root. 

Functions. — This  nerve  has  no  function  other  than  to 
supply  motion  to  the  parts  to  which  it  is  distributed.  It  is 
insensible  at  its  root,  but  receives  filaments  from  the  fifth 
in  the  cavernous  sinus,  beyond  which  point  stimulation  pro- 
duces pain  as  well  as  muscular  contractions.  The  phenomena 
sequent  upon  section  of  the  nerve  are  suggested  in  its  distri- 
bution. ( i )  There  is  ptosis,  or  dropping  of  the  upper  lid ; 
for  the  lid  is  kept  open  by  the  levator  palpebrae  superioris. 
(2)  There  is  external  strabismus,  because  the  external 
rectus  is  not  supplied  by  this  nerve  and  is  unopposed  by  the 
internal  rectus,  the  action  of  which  is  paralyzed.  Diplopia 
is  the  consequence.  (3)  There  is  inability  to  turn  the  ball 
except  in  an  outward  direction  because  the  muscles  produc- 
ing movements  on  the  vertical  and  horizontal  axes  are  de- 
prived of  innervation.  (4)  There  is  inability  to  rotate  the 
eye  in  certain  directions  on  the  antero-posterior  axis.  The 
antagonist  of  the  inferior  oblique  is  the  superior  oblique, 
the  tendency  of  which  latter  is  to  rotate  the  globe  so  as  to 
make  the  pupil  look  downward  and  outward.  When  the 
inferior  oblique  is  paralyzed  the  superior  oblique  is  unop- 
posed, it  is  impossible  to  rotate  the  ball  as  is  usual  in  side- 
wise  movements  of  the  head,  and  double  vision  is  the  result. 
(5)  There  is  slight  protrusion  of  the  whole  ball  from  relax- 
ation of  the  muscles.  (6)  The  pupil  is  dilated  and  move- 
ments of  the  iris  are  interfered  with.  Stimulation  of  the 
third  nerve  contracts  the  pupil,  but  when  it  is  cut  the  pupil 
does  not  respond  to  light.  The  ciliary  nerves  controlling 
the  movements  of  the  iris  come  from  the  ophthalmic  gang- 
lion of  the  sympathetic ;  to  this  ganglion  goes  a  branch  from 
the  third  nerve.  It  is  known  that  the  action  of  the  sympa- 
thetic cannot  be  divorced  from  that  of  the  cerebro-spinal 


28O  THE  NERVOUS  SYSTEM 

system;  and  whether  this  influence  of  the  third  nerve  is 
exerted  directly  upon  the  iris  or  indirectly,  through  the  oph- 
thalmic ganglion  is  a  matter  of  some  obscurity.  The  fact 
that  the  action  of  the  iris  is  not  instantaneous  strongly  sug- 
gests control  by  the  sympathetic. 

The  decussation  under  the  aqueduct  of  Sylvius  is  evi- 
denced by  the  reflex  contraction  of  the  pupil  on  the  opposite 
side  when  the  central  end  of  a  divided  optic  nerve  is  stimu- 
lated. The  impulse  is  reflected  through  the  third  nerve.  It 
is  not  to  be  understood,  however,  that  the  motor  oculi  is 
the  only  nerve  capable  of  influencing  movements  of  the  iris. 
Section  of  the  sympathetic  in  the  neck  contracts  the  pupil, 
even  after  section  of  the  third. 

Fourth  Nerve  (Patheticus). 

Origin. — This  is  a  purely  motor  nerve.  Its  apparent 
origin  is  behind  the  corpora  quadrigemina  from  the  valve  of 
Vieussens.  The  two  nerves  decussate  above  this  valve.  Its 
deep  origin  is  just  below  that  of  the  third  nerve  beneath  the 
aqueduct  of  Sylvius. 

Course  and  Distribution. — Emerging  from  the  valve  of 
Vieussens  the  nerve  winds  around  the  superior  peduncle  of 
the  cerebellum  and  the  crusta  immediately  above  the  pons, 
and  passes  forward  near  the  outer  wall  of  the  cavernous 
sinus  to  find  exit  from  the  cranial  cavity  by  the  sphenoidal 
fissure.  Having  entered  the  orbit,  it  runs  forward  to  be  dis- 
tributed to  the  orbital  surface  of  the  superior  oblique.  In 
the  cavernous  sinus  it  receives  fibers  from  the  ophthalmic 
division  of  the  fifth  and  from  the  sympathetic,  and  occasion- 
ally gives  off  a  branch  to  the  lachrymal  nerve. 

Function. — It  supplies  motor  power  to  the  superior  ob- 
lique muscle  alone.  Remembering  the  origin  and  attachment 
of  this  muscle  it  is  not  difficult  to  foretell  the  consequence  of 
lesions  of  the  nerve.  The  action  of  the  superior  oblique  is 
to  rotate  the  ball  upon  an  oblique  horizontal  axis  so  that 


THE  CRANIAL  NERVES  28l 

the  pupil  will  look  downward  and  outward.  This  move- 
ment cannot  be  accomplished  when  the  nerve  is  cut,  and  the 
inferior  oblique  asserts  itself  unduly  to  bring  about  an  op- 
posite effect.  The  ball  cannot  accommodate  itself  to  move- 
ments of  the  head  toward  the  shoulder,  and  double  vision 
supervenes — -unless  the  object  be  brought  in  the  involuntary 
line  of  vision  of  the  affected  eye. 

Fifth  Nerve  (Trifacial,  Trigeminus). 

The  fifth  is  analogous  to  the  spinal  nerves  (i)  in  rising 
by  two  roots,  (2)  in  having  a  ganglion  on  its  posterior  root, 
and  (3)  in  having  a  mixed  function.  The  anterior  root  is 
small  and  motor;  the  posterior  large  and  sensory. 

Origin. — Its  apparent  origin  is  from  the  side  of  the  pons 
above  the  median  line.  The  deep  origin  of  the  large,  sen- 
sory root  is  in  the  pons  immediately  below  the  floor  of  the 
fourth  ventricle  and  just  internal  to  its  marginal  boundary. 
The  small,  motor  root  rises  from  a  point  just  internal  to  the 
large  root. 

Course  and  Distribution. — The  two  roots,  taking  their 
origin  as  above  described,  pass  through  the  dura  above  the 
internal  auditory  meatus  and  run  along  the  superior  border 
of  the  petrous  portion  of  the  temporal  bone  to  a  point  near 
its  apex,  where  a  large  ganglion,  the  semilunar  or  Gasserian, 
is  developed  on  the  posterior  root  and  occupies  a  depression 
on  the  bone  for  its  reception.  The  motor  root  passes  be- 
neath the  ganglion  without  being  connected  with  it. 

The  posterior  root  will  be  first  followed  to  its  distribu- 
tion. 

From  the  anterior  surface  of  the  Gasserian  ganglion  are 
given  off  three  branches — (i)  ophthalmic,  (2)  superior 
maxillary,  (3)  inferior  maxillary.  After  the  inferior  max- 
illary has  left  the  cranial  cavity  it  receives  fibers  from  the 
small  or  motor  root,  but  the  other  branches  are  composed 
entirely  of  fibers  from  the. sensory  root. 


282  THE  NERVOUS  SYSTEM 

1.  The  Ophthalmic  Branch  passes  forward  along  the  outer 
wall  of  the  cavernous  sinus,  divides  into  three  branches — 
(a)  lachrymal,  (b)  frontal,  (c)  nasal — and  enters  the  orbit 
by  the  sphenoidal  fissure.    It  communicates  with  the  cavern- 
ous sympathetic,  third  and  sixth  nerves,    (a)  The  lachrymal 
branch,  running  along  the  outer  wall  of  the  orbit,  reaches 
the  lachrymal  gland,  gives  off  filaments  to  it  and  to  the  con- 
junctiva, and  pierces  the  tarsal  ligament  to  be  finally  dis- 
tributed  to   the   integument   of   the   upper   lid.      (b)    The 
frontal  branch  runs  along  the  upper  wall  of  the  orbit  and 
separates     into     the     supra-trochlear     and     supra-orbital 
branches.    The  former  of  these  leaves  the  orbit  in  front  and 
turns  up  over  the  bone  to  supply  the  integument  of  the 
lower  forehead ;  the  latter  traverses  the  supra-orbital  canal, 
escapes  by.  the   foramen  of  the  same  name,  and  supplies 
the  skin  as  far  back  as  the  occiput  as  well  as  the  peri- 
cranium in  the  frontal  and  parietal  regions,     (c)  The  nasal 
branch,  crossing  to  the  inner  wall  of  the  orbit,  enters  the 
anterior  ethmoidal  foramen,  passes  thus  into  the  cranium 
again,  runs  in  a  groove  on  the  cribriform  plate  of  the  eth- 
moid and  finds  exit  into  the  nose  through  a  slit  by  the  side 
of  the  crista  galli.     Here  it  gives  off  branches  which  supply 
common  sensation  to  the  mucous  membrane  of  the  fore  part 
of  the  nose,  and  then  running  in  a  groove  on  the  posterior 
surface  of  the  nasal  bone,  it  leaves  the  cavity  at  the  lower 
border  of  that  bone  to  supply  the  integument  of  the  ala  and 
tip  of  the  nose.    From  the  nasal  nerve  pass  fibers  to  the  oph- 
thalmic ganglion  and  to  the  ciliary  muscle,  iris  and  cornea. 

2.  The  Superior  Maxillary  Branch  passes  away  from  the 
Gasserian  ganglion  and  leaves  the  cranium  by  the  foramen 
rotundum.     Crossing  the  spheno-maxillary   fossa  it  enters 
the  orbit  through  the  spheno-maxillary  fissure  and  traverses 
the  infra-orbital  canal  to  emerge  upon  the  face  at  the  infra- 
orbital  foramen.     In  the  cranium  it  gives  off  a  meningeal 
branch   to   supply   the  neighboring   dura   mater.        In   the 
spheno-maxillary   fossa   it   supplies   branches    (a)    to   the 


THE  CRANIAL  NERVES  283 

integument  over  the  temporal  and  post-frontal  regions  and 
over  the  cheeks;  (b)  to  the  spheno-palatine  ganglion;  (c) 
the  posterior  superior  dental  branches  (generally  two), 
which  enter  the  posterior  dental  canals  in  the  zygomatic 
fossa,  and,  passing  forward  in  the  substance  of  the  superior 
maxilla,  give  off  twigs  to  the  fangs  of  the  molar  teeth,  sup- 
plying them  with  sensation.  In  the  infra-orbital  canal  the 
superior  maxillary  nerve  gives  off  (a)  the  middle  superior 
dental,  which  runs  downward  and  forward  in  the  outer  wall 
of  the  antrum  to  reach  the  roots  of  the  bicuspid  teeth;  (b) 
the  anterior  superior  dental,  which  likewise  runs  in  the 
outer  wall  of  the  antrum  to  supply  the  incisor  and  canine 
teeth.  After  its  exit  from  the  infra-orbital  canal  the  nerve 
divides  into  palpebral,  nasal  and  labial  branches,  which  sup- 
ply sensation  to  the  regions  indicated  by  their  names. 

3.  The  Inferior  Maxillary  Branch  after  its  exit  from  the 
cranium  is  a  mixed  nerve,  supplying  motion  to  the  muscles 
of  mastication  as  well  as  common  sensation  to  the  parts 
presently  to  be  noted,  and  special  sense  to  a  part  of  the 
tongue.  Its  large  or  sensory  root  comes  from  the  Gasserian 
ganglion  to  be  joined  just  beneath  the  base  of  the  skull  by 
the  small  motor  root  which  has  passed  -under  the  ganglion. 
Almost  immediately  this  common  trunk  divides  into  (a) 
anterior  and  (b)  posterior  branches,  but  first  gives  off  a  re- 
current meningeal  branch  and  a  branch  to  the  internal  ptcry- 
goid  muscle. 

(a)  The  anterior  of  the  two  divisions  of  the  inferior  max- 
illary nerve  receives  nearly  the  whole  of  the  motor  root  and 
divides  into  branches  which  supply  the  muscles  of  mastica- 
tion, excepting  the  internal  pterygoid  and  the  buccinator. 

(b)  The  posterior  division,  chiefly  sensory,  divides  into 
the  auriculo-temporal,  lingual  and  inferior  dental  branches. 
The  auriculo-temporal  branch  runs  backward  to  a  point  in- 
ternal to  the  neck  of  the  condyle  of  the  inferior  maxilla,  then 
passing    upward    under    the    parotid    gland    divides    into 
^branches,   which  are  distributed  to  the  external  auditory 


284  THE  NERVOUS  SYSTEM 

meatus,  parotid  gland,  integument  of  the  temporal  region 
and  of  the  ear  and  surrounding  parts.  It  communicates  with 
the  otic  ganglion.  The  lingual  branch  is  joined  by  the 
chorda  tympani,  passes  to  the  inner  side  of  the  ramus  of  the 
jaw,  crosses  Wharton's  duct,  and  is  distributed  to  the  pa- 
pillae and  mucous  membrane  of  the  tongue  and  mouth.  It 
communicates  with  the  facial  through  the  chorda  tympani, 
with  the  hypoglossal,  and  with  the  submaxillary  ganglion. 
The  inferior  dental  branch  passes  between  the  internal  lat- 
eral ligament  and  ramus  of  the  jaw  to  enter  the  inferior 
dental  foramen.  Thence  it  traverses  the  dental  canal  in  the 
inferior  maxilla  to  issue  at  the  mental  foramen.  Here  it  di- 
vides into  incisor  and  mental  branches ;  the  former  con- 
tinues in  the  bone  to  supply  the  incisor  and  canine  teeth ; 
the  latter  supplies  the  skin  of  the  chin  and  lower  lip.  In  its 
course  the  inferior  dental  gives  off  the  mylo-hyoid  (before 
entering  the  canal)  to  the  mylo-hyoid  and  anterior  belly  of 
the  digastric,  and  dental  branches  to  supply  the  molar  and 
bicuspid  teeth. 

Four  small  ganglia,  usually  classed  as  part  of  the  sympa- 
thetic system,  are  connected  with  the  three  divisions  of  the 
tri facial  nerve.  The  ophthalmic,  or  lenticular,  ganglion  is 
connected  with  the  first  division ;  the  spheno-palatine  or 
Meckel's  with  the  second ;  the  otic  and  submaxillary  with 
the  third.  All  these  receive  sensory  fibers  from  the  trifacial 
and  motor  fibers  from  various  sources. 

Functions. — It  is  seen  from  the  foregoing  description  that 
the  trifacial  is  the  great  sensory  nerve  of  the  head  and  face, 
and  the  motor  nerve  of  the  muscles  of  mastication.  The 
small,  or  motor,  division  has  properly  been  called  the  "nerve 
of  mastication."  It  is  insensible  upon  stimulation  before 
it  is  joined  by  the  third  division  of  the  sensory  root.  Its  sec- 
tion causes  paralysis  of  the  muscles  of  mastication  on  that 
side.  It  cannot  be  doubted  that  the  large  root  is  exclusively 
sensory  at  its  origin,  and  the  acuteness  of  that  sensibility,  as 
c.  g.,  in  the  teeth,  is  a  matter  of  common  observation.  Im- 


THE  CRANIAL  NERVES  285 

mediate  loss  of  sensibility  in  the  area  of  its  distribution  fol- 
lows section,  and  even  the  cornea,  which  is  normally  ex- 
quisitely sensitive,  can  be  touched  without  exciting  pain. 
Both  roots  are  usually  cut  at  the  same  time,  and  besides  a 
loss  of  motion  and  general  sensibility,  section  of  this  nerve 
produces  a  decided  effect  upon  the  eye,  the  sense  of  taste, 
deglutition  and  the  nutrition  of  the  parts  to  which  the  nerve 
is  distributed.  The  flow  of  tears  is  increased,  the  pupil  be- 
comes temporarily  contracted  and  the  ball  protrudes.  In  a 
few  hours  congestion  is  marked,  and  in  a  day  or  two  the 
cornea  sloughs  and  the  eye  is  destroyed.  Section  of  the 
fifth  before  its  lingual  branch  is  joined  by  the  chorda  tym- 
pani  from  the  facial  causes  a  loss  of  general  sensation,  but 
not  of  taste,  in  the  anterior  part  of  the  tongue ;  section  of 
the  lingual  branch  after  it  has  received  the  chorda  is  fol- 
lowed by  loss  of  general  sensation  and  of  taste.  This 
shows  that  the  special^  sensibility  distributed  to  the  tongue 
by  the  lingual  branch  of  the  fifth  is  furnished  by  the  chorda 
timpani.  The  fifth  nerve  sends  filaments  to  give  sensibility 
to  the  velum  palati.  The  reflex  act  of  deglutition  is  due  to 
impressions  carried  from  the  velum  and  neighboring  parts 
to  the  centers ;  when  the  fifth  nerve  is  cut  no  such  impres- 
sions are  conveyed  and  the  reflex  act  cannot  be  excited. 

Regarding  nutrition  it  is  noticed  that,  besides  the  slough- 
ing of  the  cornea,  there  is  also,  about  the  same  time,  the  ap- 
pearance of  ulcers  in  the  mouth  and  on  the  tongue,  and  ani- 
mals thus  experimented  upon  soon  die.  These  lesions  are 
much  less  marked  when  the  section  is  behind  the  semilunar 
ganglion.  Explanations  of  this  difference  are  not  altogether 
satisfactory,  but  it  is  rational  to  suppose  that  section  of  sym- 
pathetic fibers  when  the  nerve  is  cut  in  front  of  Gasser's 
ganglion  is  responsible  for  the  disturbances  of  nutrition ;  for 
this  is  the  system  of  nutrition,  and  changes  following  its  sec- 
tion in  other  parts  of  the  body  are  not  unlike  those  under, 
discussion.  Why,  however,  the  changes  should  be  inflam- 
matory in  character  is  not  explained  by  this  hypothesis,  un- 


286  THE  NERVOUS  SYSTEM 

less  it  be  an  explanation  to  say  that  the  inflammation  is  set 
up  by  the  impairment  of  nutrition  in  these  structures — the 
impairment  resulting  in  part  from  the  impoverished  condi- 
tion of  the  blood  as  a  consequence  of  the  inability  of  the 
animal  to  chew. 

Sixth  Nerve  (Abducens). 

Origin. — This  is  a  motor  nerve  entirely.  Its  apparent 
origin  is  from  the  lower  border  of  the  pons  in  the  groove 
separating  it  from  the  anterior  pyramid  of  the  medulla.  Its 
deep  origin  is  close  to  the  median  line  beneath  the  floor  of 
the  fourth  ventricle  a  little  below  the  motor  root  of  the  fifth. 

Course  and  Distribution. — The  nerve  enters  the  cavern- 
ous sinus,  runs  forward  to  enter  the  orbit  by  the  sphenoidal 
fissure,  passes  between  the  two  heads  of  the  external  rectus, 
and  is  distributed  to  the  ocular  surface  of  that  muscle.  In 
the  cavernous  sinus  it  receives  fibers  from  the  first  division 
of  the  fifth  and  from  the  sympathetic. 

Function. — The  function  is  indicated  in  its  distribution. 
It  is  insensible  at  its  origin.  Stimulation  produces  contrac- 
tion of  the  external  rectus ;  section  causes  paralysis  of  that 
muscle  and  consequent  internal  strabismus  and  diplopia. 

Seventh  Nerve  (Facial). 

Origin. — The  apparent  origin  of  the  seventh  is  from  the 
upper  end  of  the  medulla  in  the  groove  between  the  olivary 
and  restiform  bodies.  Its  deep  origin  is  in  the  pons  beneath 
the  floor  of  the  fourth  ventricle  a  little  external  to  the  nu- 
cleus of  the  sixth. 

Course  and  Distribution. — The  seventh  nerve  passes  out- 
ward and  forward  with  the  auditory  nerve  (on  its  inner 
side)  to  enter  the  internal  auditory  meatus.  From  their 
relative  firmness  and  texture  and  their  close  relation  here, 
the  seventh  and  eighth  nerves  have  been  called  respectively 


THE  CRANIAL  NERVES  287 

the  portio  dura  and  the  portio  mollis.  Running  between 
them  is  a  fasciculus  from  the  medulla  known  as  the  inter- 
mediary nerve  of  Wrisberg,  or  the  portio  inter  duram  et 
mollem;  most  of  its  fibers  join  the  facial  in  the  internal  audi- 
tory meatus.  The  facial  nerve  enters  the  Fallopian  aque- 
duct at  the  bottom  of  the  meatus  and  follows  it  to  issue  at 
the  stylo-mastoid  foramen,  runs  forward  in  the  substance  of 
the  parotid  gland  and  divides  behind  the  ramus  of  the  jaw 
into  temp or -o- -facial  and  cervico -facial  branches. 

Its  branches  of  communication  are  numerous.  ( i )  In  the 
internal  auditory  meatus  it  communicates  with  the  auditory 
nerve;  (2)  in  the  aqueductus  Fallopii  with  the  otic  and 
spheno-palatine  ganglia,  with  the  sympathetic  and  with  the 
auricular  branch  of  the  pneumogastric ;  (3)  after  leaving 
the  stylo-mastoid  foramen,  with  the  fifth,  ninth,  tenth  and 
sympathetic. 

Its  branches  of  distribution  are  also  quite  numerous,  (i) 
In  the  aqueductus  Fallopii  it  gives  off  (a)  the  tympanic 
branch  to  the  stapedius  muscle,  and  (b]  the  chorda  tympani, 
which  passes  through  the  cavity  of  the  tympanum  and 
emerges  by  a  foramen  at  the  inner  end  of  the  Glasserian 
fissure  to  go  to  the  lingual  branch  of  the  fifth.  (2)  At  its 
exit  from  the  stylo-mastoid  foramen  it  gives  off  (a)  a 
posterior  auricular  branch  which,  receiving  a  filament  from 
the  auricular  branch  of  the  tenth,  is  distributed  to  the  retra- 
hens  aurem  and  the  occipital  portion  of  the  occipito-fron- 
talis;  (6)  a  digastric  branch  to  the  posterior  Ipelly  of  the  di- 
gastric muscle;  (c)  a  stylo-hyoid  branch  to  the  muscle  of 
that  name.  (3)  On  the  face  it  divides  into  (a)  a  temporo- 
facial  branch,  which  is  distributed  to  the  muscles  over  the 
temple  and  upper  face;  and  (b)  a  cervico-facial  branch, 
which  is  distributed  to  the  lower  face  and  upper  cervical 
region. 

Functions. — This  is  the  motor  nerve  of  the  muscles  of  ex- 
pression, of  the  platysma,  buccinator,  digastric  (posterior 
belly),  stylo-hyoid,  the  muscles  of  the  external  ear  and  the 


288  THE  NERVOUS  SYSTEM 

stapedius.  Communicating  freely  with  the  fifth,  it  also  con- 
tains sensory  fibers,  but  it  is  in  all  probability  insensible  at 
its  root.  Its  section  causes  paralysis  of  the  muscles  which  it 
supplies,  but  no  marked  changes  in  sensation.  The  branches 
to  the  otic  and  spheno-palatine  ganglia  in  the  aqueductus 
Fallopii  constitute  their  motor  roots;  the  branch  given  off 
in  this  situation  to  the  tenth  supplies  it  with  motor  filaments, 
and  probably  also  here  pass  sensory  fibers  from  the  tenth  to 
the  seventh.  In  facial  paralysis  when  the  lesion  is  in  the 
aqueductus  Fallopii  or  behind  it,  there  is  paralysis  also  of 
the  muscles  of  the  palate  and  uvula,  the  uvula  is  drawn  "to 
the  opposite  side  and  there  is  trouble  in  deglutition.  The 
fibers  to  the  azygos  uvulae  and  levator  palati  pass  from  the 
aqueductus  Fallopii  through  Meckel's  ganglion. 

The  effect  of  paralysis  of  the  facial  upon  the  superficial 
muscles  of  the  face  is  suggested  in  its  distribution.  The 
brow  cannot  be  corrugated;  the  eye  is  constantly  open  and 
there  may  be  consequent  inflammation  from  exposure ;  the 
nostril  cannot  be  dilated,  and  inspiration  and  possibly  ol fac- 
tion are  interfered  with ;  the  cheek  is  flaccid ;  the  lips  are  im- 
mobile and  saliva  may  flow  from  that  corner  of  the  mouth ; 
the  buccinator  is  paralyzed,  and  there  is  often  great  diffi- 
culty in  mastication  because  of  the  accumulation  of  food  be- 
tween the  cheek  and  the  teeth ;  the  unopposed  action  of  the 
muscles  of  the  opposite  side  greatly  distort  the  facial  fea- 
tures, the  affected  side  being  quite  expressionless.  Facial 
monoplegia  is  common ;  facial  diplegia  is  very  uncommon. 

The  Chorda  Tympani. — This  branch  of  the  seventh  is  con- 
cerned especially  in  gustation.  The  fibers  of  which  it  is 
composed  undoubtedly  come  from  nerve  of  Wrisberg.  Sec- 
tion of  the  seventh  involving  also  the  nerve  of  Wrisberg 
causes  not  only  facial  palsy  but  also  a  loss  of  the  sense  of 
taste  in  the  anterior  two-thirds  of  the  tongue.  The  sense  of 
taste  will  receive  later  notice. 


THE  CRANIAL  NERVES  289 

Eighth  Nerve  (Auditory). 

Origin. — This  is  a  nerve  of  special  sense.  Its  apparent 
origin  is  by  two  roots — one  from  the  groove  between  the 
olivary  and  restiform  bodies  at  the  lower  border  of  the  pons, 
the  other  coming  around  the  upper  end  of  the  restiform 
body  to  join  the  first  in  the  groove.  The  deep  origin  of  the 
two  roots  is  different.  That  of  the  median  root  is  the  dor- 
sal auditory  nucleus  in  the  floor  of  the  fourth  ventricle; 
that  of  the  lateral  root  is  mainly  from  the  ventral  auditory 
nucleus  in  front  of  the  restiform  body  between  the  two 
roots. 

Course  and  Distribution. — Crossing  the  posterior  border 
of  the  middle  peduncle  of  the  cerebellum,  it  enters  the  in- 
ternal auditory  meatus  in  company  with  the  facial  nerve  and 
the  nerve  of  Wrisberg.  At  the  bottom  of  the  meatus  it  re- 
ceives fibers  from  the  seventh,  and  divides  into  branches 
which  pass  to  the  cochlea,  semi-circular  canals  and  vestibule. 

Function. — This  nerve  receives  and  conveys  to  the  brain, 
impressions  produced  by  sound  waves;  it  is  the  nerve  of 
hearing  and  is  in  all  probability  not  sensible  to  stimulation 
in  any  other  way. 

Ninth  Nerve  (Glosso-pharyngeal). 

Origin. — The  apparent  origin  of  this  nerve  is  from  the 
upper  part  of  the  medulla  in  the  groove  between  the  olivary 
and  restiform  bodies.  Its  deep  origin  is  in  the  lower  part  of 
the  floor  of  the  fourth  ventricle  above  the  nucleus  of  the 
tenth. 

Course  and  Distribution. — Leaving  the  skull  by  the  jugu- 
lar foramen,  it  passes  forward  between  the  internal  jugu- 
lar vein  and  the  internal  carotid  artery,  descends  in  front  of 
the  latter  to  the  lower  border  of  the  stylo-pharyngeus  where 
it  curves  inward,  runs  beneath  the  hyoglossus,  and  is  distrib- 

19 


290  THE  NERVOUS  SYSTEM 

uted  to  the  fauces,  posterior  third  of  the  tongue,  and  the 
tonsil. 

It  communicates  with  the  seventh,  tenth  and  sympathetic. 

Its  branches  of  distribution  go  to  the  mucous  membrane 
and  muscles  of  the  pharynx,  the  stylo-pharyngeus,  the  ton- 
sil and  soft  palate,  the  circumvallate  papillae  and  the  mucous 
membrane  at  the  base  and  side  of  the  tongue  and  on  the  an- 
terior surface  of  the  epiglottis.  Some  of  its  branches  join 
branches  from  the  pharyngeal  and  external  laryngeal 
branches  of  the  pneumogastric  to  form  the  pharyngeal 
plexus. 

Functions. — It  is  the  nerve  of  sensation  to  the  pharynx 
and  fauces  and  a  nerve  of  taste  to  the  base  of  the  tongue. 
Its  sensibility  at  its  root  is  dull,  but  stimulation  produces  no 
motion.  Although  this  nerve  is  distributed  to  the  mucous 
membrane  over  the  base  of  the  tongue,  palate  and  pharynx, 
these  parts  receive  the  greater  portion  of  their  general  sen- 
sibility from  filaments  of  the  fifth,  and  section  of  the  ninth 
produces  no  marked  effect  upon  the  reflex  phenomena  of 
deglutition.  The  sense  of  taste  is  distributed  to  the  anterior 
two-thirds  of  the  tongue  by  the  chorda  tympani,  and  it  has 
nothing  to  do  with  general  sensation,  while  the  glosso- 
pharyngeal,  endowing  the  posterior  third  with  gustatory 
power,  also  furnishes  to  it  a  degree  of  general  sensibility. 

Tenth  Nerve  (Pneumogastric,  Vagus). 

Origin. — This  is  a  mixed  nerve.  Its  apparent  origin  is 
from  the  groove  between  the  olivary  and  restiform  bodies 
below  the  ninth!  Its  deep  origin  is  in  the  floor  of  the  fourth 
ventricle  just  below  that  of  the  glosso-pharyngeal. 

Course  and  Distribution. — As  it  leaves  the  cranium  by  the 
jugular  foramen  it  presents  a  ganglionic  enlargement,  the 
jugular  ganglion,  or  ganglion  of  the  root,  just  below  which 
it  is  joined  by  the  accessory  portion  of  the  spinal  accessory. 
Below  the  junction  is  a  second  ganglion,  the  ganglion  of  the 


THE  CRANIAL  NERVES  291 

trunk.  The  accessory  part  of  the  eleventh  passes  through 
this  ganglion,  and  below  unites  with  the  vagus  trunk  to  pass 
chiefly  into  its  pharyngeal  and  superior  laryngeal  branches. 
The  pneumogastric  passes  down  the  neck  behind  and.  be- 
tween the  internal  jugular* vein  and  the  internal  and  com- 
mon carotid  arteries,  and  sends  motor  and  sensory  fibers  to 
the  organs  of  voice  and  respiration,  and  motor  fibers  to  the 
pharynx,  esophagus,  stomach  and  heart. 

The  branches  of  the  pneumogastric  are  numerous,  (i) 
In  the  jugular  fossa  it  gives  off  (a)  a  meningeal  branch  to 
the  dura  mater  of  the  posterior  fossa  of  the  skull;  (b)  an 
auricular  branch  which,  traversing  the  substance  of  the  tem- 
poral bone,  emerges  by  the  auricular  fissure  to  supply  the  in- 
tegument of  the  back  part  of  the  pinna  and  external  auditory 
meatus.  (2)  In  the  neck  it  gives  off  (a)  a  pharyngeal 
branch,  which  consists  mainly  of  fibers  from  the  accessory 
portion  of  the  eleventh  and  is  the  chief  motor  nerve  of  the 
pharynx  and  soft  palate;  (b)  a  superior  laryngeal  branch, 
which  also  consists  mainly  of  fibers  from  the  accessory  part 
of  the  eleventh  and  is  the  chief  sensory  nerve  of  the  larynx ; 
it  also  animates  the  crico-thyroid  muscle;  (c)  a  recurrent 
laryngeal  branch,  which,  on  the  right  side,  winds  round  the 
subclavian  artery  and,  on  the  left,  round  the  aorta  to  re- 
turn to  the  muscles  of  the  larynx  whose  motor  nerve  it  is ; 
(d)  cervical  cardiac  branches,  which  communicate  with  the 
cardiac  branches  of  the  sympathetic  and  pass  to  the  deep 
cardiac  plexus.  (3)  In  the  thorax  it  gives  off  (a)  thoracic 
cardiac  branches,  which  pass  to  the  deep  cardiac  plexus ;  (b) 
anterior  pulmonary  branches,  which  go  to  the  roots  of  the 
lungs  in  front;  (c)  posterior  pulmonary  branches,  which  go 
to  the  roots  of  the  lungs  behind  and  send  some  filaments  to 
the  pericardium;  filaments  from  (b)  and  (c)  follow  the  air 
passages  through  the  lungs;  (d)  esophageal  branches,  which 
unite  with  fibers  from  the  opposite  nerve  to  form  the  esopha- 
geal plexus.  (4)  In  the  abdomen  are  the  gastric  branches; 
those  from  the  left  nerve  are  distributed  to  the  anterior 


292  THE  NERVOUS  SYSTEM 

surface  of  the  stomach,  and  those  from  the  right  to  the  pos- 
terior ;  the  right  vagus  is  also  distributed  to  the  liver,  spleen, 
kidneys  and  entire  small  intestine. 

Throughout  its  whole  course  the  pneumogastric  communi- 
cates with  other  nerves,  especially  the  sympathetic. 

Functions. — The  root  of  the  tenth  in  the  medulla  is 
purely  sensory,  but  the  nerve  communicates  with  at  least  five 
motor  nerves,  and  is  distributed  to  mucous  membranes  and 
to  voluntary  and  involuntary  muscle  tissue.  The  auricular 
branches  contain  both  motor  and  sensory  fibers,  and  their 
function  is  indicated  in  their  distribution.  The  pharyngeal 
branches  are  mixed,  receiving  motor  filaments  from  the 
spinal  accessory.  Sensibility  is  supplied  to  the  pharynx  not 
by  this  nerve  alone,  but  by  the  branches  of  the  fifth  and 
probably  of  the  ninth ;  indeed  it  seems  that  the  pharyngeal 
branches  of  the  tenth  have  little  to  do  with  the  reflex  phe- 
nomena of  deglutition.  The  superior  laryngeal  branches, 
mainly  sensory,  supply  also  motor  power  to  the  crico-thy- 
roids.  Stimulation  of  the  filaments  of  these  branches  pre- 
vents the  entrance  of  foreign  bodies  into  the  larynx  by  reflex 
closure  of  the  glottis,  and  also  excites  movements  of  deglu- 
tition. Their  section  produces  hoarseness.  The  recurrent, 
or  inferior  laryngeal,  branches,  chiefly  motor,  supply  the 
muscular  tissue  of  the  upper  esophagus  and  trachea,  as  well 
as  the  muscles  of  the  larynx.  Section  of  them  causes  em- 
barrassed phonation,  though  the  fibers  thus  influencing  the 
vocal  sounds  come  to  the  recurrent  laryngeal  from  the  spinal 
accessory.  The  uses  of  the  cardiac  branches  have  been  no- 
ticed under  discussion  of  the  heart's  action.  The  pulmonary 
branches  are  both  motor  and  sensory  and  go  to  the  lower 
trachea,  the  bronchi  and  lung  substance.  Section  of  the 
tenth  destroys  the  sensibility  of  the  mucous  membrane  of 
the  trachea  and  bronchi  and  the  contractile  power  of  the 
muscular  fibers  of  the  tubes.  The  esophageal  branches  are 
mixed,  though  motor  fibers  predominate.  Food  will  not  pass 


THE  CRANIAL  NERVES  293 

readily  into  the  stomach  on  section  of  the  tenth  because  of 
the  absence  of  muscular  contractions  in  the  esophagus. 

Influence  of  the  Vagus  on  Respiration. — Section  of  both 
these  nerves  temporarily  increases  the  number  of  respira- 
tions which  soon,  however,  become  exceedingly  slow  until 
death  ensues.  Inspiration  is  very  profound — indeed  so  pro- 
found as  to  produce  rupture  of  some  of  the  pulmonary 
capillaries  with  consequent  hemorrhage  and  coagulation  of 
the  blood  and  consolidation  of  the  lung  in  part  or  whole. 
Section  of  only  one  of  the  vagi  is  not  usually  followed  by 
death.  Further  notice  of  the  relation  of  the  pneumogastric 
to  respiration  is  given  elsewhere. 

Influence  of  the  Vagus  on  the  Stomach,  Intestine  and 
Liver. — Stimulation  of  the  pneumogastric  causes  contraction 
of  the  stomach;  but  since  the  contraction  is  not  immediate, 
the  impulse  is  probably  carried  to  it  by  fibers  of  the  sympa- 
thetic running  with  the  gastric  branches  of  the  tenth.  When 
the  vagus  is  cut  during  digestion  in  the  stomach  the  contrac- 
tions of  the  muscular  wall  are  impaired  and  the  sensibility  of 
the  organ  is  abolished.  Secretion  is  interfered  with,  but  not 
stopped. 

Section  of  the  vagus  seems  also  to  impair  intestinal  secre- 
tion and  movements,  but  it  is  not  improbable  that  this  is  be- 
cause sympathetic  fibers  joining  the  vagus  high  in  the  neck 
are  distributed  with  it  to  the  intestine. 

Simple  division  of  the  pneumogastrics  inhibits  the  forma- 
tion of  glycogen  in  the  liver;  but  when  the  central  ends  of 
the  cut  nerves  are  stimulated  there  is  an  increased  pro- 
duction of  sugar  even  to  the  point  of  glycosuria.  The  irri- 
tation is  probably  reflected  through  the  sympathetic;  indeed 
it  is  not  supposed  that  the  vagi  are  concerned  in  the  glyco- 
genic  function  of  the  liver,  except  reflexly ;  its  section  only 
prevents  the  conduction  cephalad  of  the  impressions  which 
usually  give  rise  to  a  secretion  of  glycogen. 

The  connection  of  the  vagus  with  the  kidneys,  spleen  and 
suprarenal  capsules  is  obscure. 


294  THE  NERVOUS  SYSTEM 

Eleventh  Nerve  (Spinal  Accessory). 

Origin. — This  nerve  consists  of  a  cranial  portion,  acces- 
sory to  the  tenth,  and  a  spinal  portion.  The  apparent  origin 
of  the  cranial  root  is  from  the  side  of  the  medulla  just  below 
the  vagus.  Its  deep  origin  is  in  the  medulla  to  the  posterior 
and  outer  side  of  the  nucleus  of  the  ninth.  The  apparent 
origin  of  the  spinal  portion  is  by  several  filaments  from  the 
side  of  the  cord  as  low  down  as  the  sixth  cervical  nerve.  Its 
deep  origin  is  from  a  column  of  cells  in  the  anterior  cornu 
of  gray  matter  of  the  cord. 

Course  and  Distribution  (Accessory  Portion). — Passing 
out  to  the  jugular  foramen  it  is  joined  by  the  spinal  portion, 
and  sends  a  few  filaments  to  the  ganglion  of  the  root  of  the 
tenth ;  then  leaving  the  spinal  portion  it  finds  exit  from  the 
cranium  by  the  jugular  foramen,  passes  over  the  ganglion  of 
the  trunk  of  the  tenth  (adherent  to  it),  and  is  continued 
chiefly  in  the  pharyngeal  and  superior  laryngeal  branches  of 
that  nerve  (Gray),  but  in  the  recurrent  laryngeal  as  well. 

Spinal  Portion. — Running  upward  between  the  two  roots 
of  the  spinal  nerves  the  spinal  portion  enters  the  cranial  cav- 
ity by  the  foramen  magnum,  passes  outward  to  the  jugular 
foramen,  where  it  joins  the  accessory  portion  to  separate 
from  it  on  passing  through  that  foramen.  After  leaving  the 
skull  it  takes  a  course  backward,  pierces  the  sterno-mastoid, 
crosses  the  occipital  triangle  and  terminates  in  the  trapezius. 
It  gives  branches  to  the  sterno-mastoid  and  to  the  cervical 
plexus. 

Functions. — Both  roots  of  this  nerve  are  purely  motor,  but 
communication  with  other  nerves  gives  it  a  degree  of  sensi- 
bility. The  fibers  from  the  medulla  (accessory)  go  exclu- 
sively to  the  muscles  of  the  larynx  and  pharynx,  while 
those  from  the  cord  (spinal)  go  exclusively  to  the  sterno- 
mastoid  and  trapezius;  and  section  of  either  root  separately 
is  followed  by  phenomena  corresponding  to  these  facts. 
When  both  roots  are  divided  there  is  loss  of  voice,  disturb- 


THE  CRANIAL  NERVES  295 

ance  of  deglutition,  loss  of  cardiac  inhibition  and  partial 
paralysis  of  the  sterno-mastoid  and  trapezius.  The  loss  of 
voice  and  disturbance  in  deglutition  are  explained  by  the  dis- 
tribution of  the  fibers  of  the  eleventh. with  the  pharyngeal 
and  laryngeal  branches  of  the  tenth.  The  loss  of  the  power 
of  the  vagus  to  inhibit  cardiac  action  is  because  the  fibers  of 
the  tenth  which  convey  the  inhibitory  impulses  are  received 
from  the  spinal  accessory.  The  sterno-mastoid  and  trape- 
zius are  only  partially  paralyzed  because  they  receive  motor 
fibers  also  from  the  cervical  plexus. 

Twelfth  Nerve  (Hypoglossal). 

Origin. — This  nerve  supplies  motion  to  the  tongue.  Its 
apparent  origin  is  by  1015  filaments  in  the  groove  between 
the  anterior  pyramid  of  the  medulla  and  the  olivary  body. 
Its  deep  origin  is  in  the  floor  of  the  fourth  ventricle  under 
the  lower  border  of  the  fasciculus  teres. 

Course  and  Distribution. — The  nerve  passes  through  the 
anterior  condyloid  foramen  in  two  bundles  which  unite  to 
form  a  common  trunk  below.  Running  downward  in  com- 
pany with  the  internal  carotid  artery  and  internal  jugular 
vein,  it  reaches  a  point  opposite  the  angle  of  the  jaw,  then 
runs  forward,  crosses  the  external  carotid,  lies  on  the  hyo- 
glossus  and  is  continued  forward  in  the  genio-hyoglossus 
to  the  tip  of  the  tongue. 

It  communicates  with  the  tenth,  sympathetic,  .first  and  sec- 
ond cervical  and  the  lingual  branch  of  the  fifth. 

Its  branches  of  distribution  are  ( I )  meningeal  to  the  dura 
mater  in  the  posterior  fossa  of  the  skull;  (2)  descendens 
hypoglossi,  which  running  downward  across  the  sheath  of 
the  great  vessels,  meets  branches  of  the  second  and  third 
cervical  nerves  to  form  a  loop  from  which  are  supplied  the 
sterno-hyoid,  the  omo-hyoid  and  the  sterno-thyroid  muscles ; 
(3)  thyro-hyoid  to  the  muscle  of  that  name;  (4)  muscular 
to  the  muscular  substances  of  the  tongue  and  to  the  stylo- 


296  THE  NERVOUS  SYSTEM 

glossus,  hyoglossus,  genio-hyoid  and  genio-hyoglossus  mus- 
cles. 

Functions. — This  nerve  posseses  no  sensibility  at  its  root, 
but  receives  sensory  fibers  from  anastomoses  with  other 
nerves.  Its  stimulation,  therefore,  causes  movements  of  the 
tongue  and  some  pain.  Section  of  both  nerves  causes  difficult 
deglutition,  loss  of  power  over  the  tongue  and  consequent 
disturbances  in  mastication  and  articulation.  When  the 
twelfth  is  affected  in  hemiplegia  the  tongue,  on  protrusion, 
deviates  to  the  affected  side  because  it  is  pushed  out  by  the 
genio-hyoglossus. 

It  will  be  seen  from  the  foregoing  that,  classified  accord- 
ing to  their  properties  at  their  roots,  the  L,  II.  and  VIII. 
are  nerves  of  special  sense ;  the  III.,  IV.,  VI.,  XI  and  XII. 
are  motor;  the  X.  is  sensory;  and  the  V.,  VII.  and  IX.  are 
mixed.  It  is  to  be  remembered,  however,  that  most  of  these 
(excepting  the  nerves  of  special  sense)  are  mixed  in  their 
distribution  by  reason  of  the  reception  of  fibers  from  other 
nerves.  The  term  "mixed"  in  the  above  classification  is  used 
as  meaning  the  association  of  special  sensory  fibers  with 
motor  or  common  sensory  fibers  as  well  as  the  associa- 
tion of  these  latter  with  each  other.  The  VII.  is  classed  as 
a  mixed  nerve  only  by  allowing  the  intermediary  nerve  of 
Wrisberg  is  to  be  considered  a  part  of  it.  Its  own  proper 
root  is  purely  motor. 

THE  SPINAL  NERVES. 

The  spinal  nerves,  thirty-one  on  each  side,  are  so  called 
from  the  fact  that  they  originate  in  the  spinal  cord  and  es- 
cape from  the  spinal  canal  by  the  intervertebral  foramina. 
Eight  pairs  come  from  the  cervical  region  of  the  column, 
twelve  from  the  dorsal,  five  from  the  lumbar,  five  from  the 
sacral,  and  one  from  the  coccygeal.  They  are  numbered  ac- 
cording to  their  foramina  of  exit. 

Each  nerve  rises  by  two  roots — an  anterior  which  can  be 


THE  SPINAL   NERVES 


297 


traced  to  the  anterior  cornu  of  gray  matter  and  a  posterior 
which  goes  (apparently)  to  the  posterior  cornu — and  these 
emerge  respectively  from  the  antero-lateral  and  postero-lat- 
eral  fissures  of  the  cord.  .Before  leaving  the  spinal  canal 
these  two  roots  join  to  pass  through  the  corresponding  in- 
tervertebral  foramen  as  a  single  trunk  which,  however,  just 
beyond  that  foramen  divides  into  anterior  and  posterior 


B. 


FIG.  87. 

A,  bipolar  cell  from  spinal  ganglion  of  a  4%  weeks'  embryo  (after  His),  n, 
nucleus;  the  arrows  indicate  the  direction  in  which  the  nerve  processes  grow, 
one  to  the  spinal  cord,  the  other  to  the  periphery.  B,  a  cell  from  the  spinal  gan- 
glion of  the  adult;  the  two  processes  have  coalesced  to  form  a  T-shaped  junc- 
tion. (Kirkes.) 

branches  to  be  distributed  to  the  anterior  and  posterior  parts 
of  the  body. 

The  posterior  root  (inside  the  spinal  canal)  is  sensory, 
and  has  a  ganglion  developed  upon  it.  The  fibers  of  the 
posterior  root  are  outgrowths  of  cells  in  the  ganglion  of  that 
root,  as  indicated  in  Fig.  87.  This  accounts  for  the  arborisa- 
tion of  the  different  fibers  around  cells  in  the  cord  instead 
of  an  actual  connection  with  them.  These  facts  should  not 
be  lost  sight  of  though  it  is  customary  to  speak  of  an  efferent 


298  THE  NERVOUS  SYSTEM 

fiber  as  passing  directly  to  a  cord  cell  itself.  The  anterior 
root  is  entirely  motor  except  for  a  degree  of  "recurrent" 
sensibility  which  is  due  to  the  presence  in  it  of  posterior  root 
fibers  which  have  passed  backward  from  the  point  of  junc- 
tion of  the  two  probably  to  supply  the  membranes  of  the 
cord.  The  common  trunk  is,  of  course,  mixed,  as  are  the 
anterior  and  posterior  branches  passing  from  it. 

These  spinal  nerves  are  distributed  to  the  muscles  of  the 
trunk  and  extremities,  to  the  integument  of  almost  the  entire 
body  and  to  some  mucous  membranes;  and  from  what  has 
been  said  in  speaking  of  the  cord  about  the  connection  be- 
tween it  and  these  nerves,  and  their  connection  through  it 
with  the  higher  centers,  it  is  evident  that  they  are  most  im- 
portant factors  which,  acting  under  the  guidance  of  the  sen- 
sorium,  on  the  one  hand,  tell  of  the  condition  of  the  or- 
ganism— its  relations  and  environments — and,  on  the  other, 
control  the  voluntary  movements  of  the  body. 

The  spinal  nerve  fibers  come  in  part  directly  from  the 
brain  and  in  part  from  the  gray  cells  of  the  cord. 

THE  SYMPATHETIC  SYSTEM. 

The  sympathetic  has  been  separated  from  the  cerebro- 
spinal  system  only  for  the  sake  of  convenience.  The 
former  sends  filaments  to  the  latter  and  receives  both  motor 
and  sensory  .fibers  in  return,  while  the  cooperation  of  the  two 
systems,  regulating  in  harmony  all  the  physiological  pro- 
cesses going  on  in  the  body,  is  too  evident  to  be  questioned. 

The  sympathetic  system  is  remarkable  for  the  number  of 
ganglia  connected  with  it.  These  may  be  divided  into  (a) 
those  along  the  vertebral  column,  as  the  thoracic,  (b)  those 
in  close  proximity  to  the  viscera  and  from  which  those  vis- 
cera are  to  be  directly  supplied,  as  the  semilunar,  and  (r)  ter- 
minal ganglia  which  the  fibers  reach  just  before  final  distri- 
bution, as  the  cardiac,  intestinal,  etc.  The  sympathetic  is, 
therefore,  frequently  known  as  the  ganglionic  system. 


THE  SYMPATHETIC  SYSTEM  299 

Arrangement. — There  is  on  each  side  of  the  spinal  column, 
extending  from  the  lenticular  ganglion  above  to  the  gang- 
lion impar  below,  a  chain  of  ganglia  all  of  which  are  united 
to  each  other  and  to  the  ganglia  of  the  opposite  chain  by 
commissural  fibers.  From  these  ganglia  go  fibers  to  form 
numerous  plexuses  and  to  be  distributed  to  the  various  parts. 
In  the  skull  there  are  four  of  these  ganglia,  the  otic,  oph- 
thalmic, submaxillary  and  spheno-palatine  or  Meckel's;  in 
the  cervical  region  there  are  three;  in  the  dorsal  twelve;  in 
the  lumbar  four ;  in  the  sacral  four  or  five ;  and  in  front  of 
the  coccyx  the  single  ganglion  impar. 

Connections  between  the  cranial  nerves  and  cranial  sym- 
pathetic ganglia  have  already  been  noted. 

The  cervical  ganglia  are  of  special  interest  as  furnishing 
the  chief  sympathetic  supply  to  the  heart. 

The  thoracic  or  dorsal  ganglia  give  rise  to  the  sympa- 
thetic supply  for  the  great  abdominal  viscera.  From  the 
sixth,  seventh,  eighth  and  ninth  springs  the  great  splanchnic 
nerve,  which  passes  through  the  diaphragm  to  the  semilunar 
ganglion.  This  is  the  largest  of  the  sympathetic  ganglion, 
and  is  sometimes  called  the  abdominal  brain.  It  has  been  in- 
accurately called  the  center  of  the  sympathetic  system.  The 
two  ganglia  occupy  positions  on  opposite  sides  of  the  celiac 
axis,  and  give  rise  to  fibers  which  supply  most  of  the  abdom- 
inal viscera.  The  tenth  and  eleventh  thoracic  ganglia  give 
rise  to  the  lesser  splanchnic  nerve.  From  the  last  thoracic 
springs  the  renal  splanchnic  nerve.  The  radiating  fibers 
from  the  semilunar  ganglia  form  the  solar  plexuses  for  the 
two  sides. 

The  lumbar  ganglia  give  off  fibers  to  form  the  aortic  lum- 
bar and  hypogastric  plexuses. 

The  sacral  and  coccygeal  ganglia  supply  the  pelvic  vessels. 

Properties. — The  ganglia  and  nerves  are  slightly  sensitive. 
Contraction  of  involuntary  muscular  tissue  follows  stimula- 
tion— not  immediately,  but  after  a  considerable  interval,  and 
the  subsequent  relaxation  is  tardy.  Some  of  the  ganglia  are 


3OO  THE  NERVOUS  SYSTEM 

dependent  for  power  upon  their  fibers  from  the  cerebro- 
spinal  system,  while  others  seem  capable  of  acting  indepen- 
dently, at  least  for  a  time. 

Functions. — Little  is  known  of  the  functions  of  the  sym- 
pathetic except  as  regards  efferent  fibers.  They  are  dis- 
tributed in  general  to  the  non-striped  musculature  of  the  cir- 
culatory apparatus  and  of  the  viscera,  to  secreting  glands 
and  to  the  heart.  The  heart  furnishes  the  only  example  of  a 
direct  sympathetic  supply  to  striated  muscle.  The  sympa- 
thetic has  a  very  definite  effect  upon  secretion,  nutrition  and 
the  local  production  of  heat.  Section  of  the  sympathetic 
fibers  going  to  any  part  causes  hyperemia,  an  increased 
amount  of  secretion  (sweat,  e.  g.),  and  a  rise  of  temperature 
in  that  part.  The  last  two  conditions  are  caused  by  the  first, 
and  it  in  turn  is  due  to  a  paralysis  of  the  muscular  coat  of  the 
vessels,  allowing  an  abrogation  of  their  usual  tonic  condition 
and,  consequently,  dilatation  and  an  increased  amount  of 
blood  with  exaggerated  nutritive  activity.  This  statement 
confronts  us  with  the  question  of  vaso-motor  action. 

Vaso-motor  Phenomena. — By  vaso-motor  nerves  is  meant 
those  fibers  which  convey  to  the  muscular  coat  of  the  vessel 
walls  impulses  causing  them  to  contract  and  decrease  the 
caliber,  or  to  relax  and  increase  it.  Those  causing  contrac- 
tion are  called  vaso-constrictors ;  those  causing  relaxation 
vaso-dilators.  It  is  mainly  through  the  operation  of  vaso- 
motor  nerves  that  the  sympathetic  system  influences  nutri- 
tion in  a  particular  part,  though  all  vaso-motor  fibers  are  not 
confined  to  the  sympathetic  cords.  However,  it  is  not 
through  the  operation  of  the  vaso-motor  nerves  alone  that 
the  sympathetic  lays  claim  to  be  the  "system  of  nutrition," 
for  all  the  parts  to  which  its  other  fibers  are  distributed  con- 
tribute also  very  materially  to  nutrition,  though  perhaps  in 
not  so  direct  a  manner  as  do  the  muscular  coats  of  the  ar- 
teries. While  intestinal  peristalsis,  the  secretion  of  many 
glands,  as,  for  example,  the  production  of  glycogen,  bile,  etc., 
cannot  be  shown  to  be  absolutely  dependent  on  sympathetic 


THE  SYMPATHETIC  SYSTEM  3OI 

connections,  yet  all  these  processes — nutritive  in  nature — 
have  their  normal  activity  seriously  impaired  by  with- 
drawal of  the  sympathetic  influence. 

The  chief  vaso-motor  center  is  in  the  medulla,  though  ac- 
cessory centers  exist  also  in  the  cord ;  all  vaso-motor  fibers 
pass  out  from  these  centers  and  leave  the  cerebro-spinal 
axis  with  the  cranial  or  spinal  nerves. 

The  most  usual  mode  of  action  of  the  vaso-motor  nerves 
is  reflex,  as  when  the  mucous  membrane  of  the  stomach  be- 
comes hyperemic  upon  the  introduction  of  food ;  or  when  the 
salivary  secretion  increases  during  mastication,  or  even 
sometimes  at  the  sight  or  thought  of  food;  or  when  emo- 
tions are  evidenced  by  paling  or  blushing. 

Raising  blood-pressure  by  stimulating  the  vaso-constric- 
tors  and  lowering  it  by  stimulating  the  vaso-dilators  are  sim- 
ply mechanical  results,  and  require  no  comment. 

Sleep. — Sleep  is  closely  associated  with  vaso-motor  action. 
Every  part  of  the  body  has  a  function  to  perform,  but  it 
must  have  some  rest  from  that  performance  or  it  will  begin 
to  act  inefficiently  and  finally  cease  altogether.  For  most 
organs  these  periods  of  rest  occur  at  approximately  uniform 
intervals,  as  in  case  of  the  stomach,  heart  or  respiratory 
muscles ;  but  notably  in  case  of  the  involuntary  muscles  these 
periods  of  repose  have  no  regularity — i.  e.,  a  person  exer- 
cises them  at  no  regular  time  except  by  accident  of  occupa- 
tion or  otherwise.  But,  in  any  case,  there  comes  a  time 
when  repose  must  be  had,  for  during  activity  the  destructive 
processes  far  exceed  the  constructive,  and  in  order  for  the 
balance  to  be  preserved  there  must  be  a  time  when  the  op- 
posite is  true. 

Now  we  may  say  that  it  is  the  function  of  the  brain  to  fur- 
nish consciousness — if  we  can  allow  that  consciousness  em- 
braces all  the  various  manifestations  of  nerve  force  peculiar 
to  the  brain.  For  the  brain  to  suspend  this  function  at  fre- 
quent intervals  like  the  heart  (e.  g.)  would  be  manifestly  im- 
possible if  one  is  to  do  any  consecutive  work  depending  upon 


302  THE  NERVOUS  SYSTEM 

this  organ.  The  brain  works  longer,  and  must,  therefore, 
rest  longer  at  a  time  than  most  of  the  other  organs  of  the 
body.  True,  so  far  as  the  voluntary  muscles  are  concerned 
they  rest  best  probably  when  the  brain  is  resting,  but  the  lat- 
ter condition  is  not  a  necessary  one  for  the  maintenance  of 
their  physiological  integrity.  This  repose  of  the  brain — this 
temporary  abolition  of  the  cerebral  functions— is  sleep. 
While,  of  course,  the  activity  of  that  organ  during  wakeful- 
ness  may  be  increased  or  diminished  by  volition,  and  it  may, 
therefore  rest  from  a  comparative  standpoint — as  when  one 
ceases  to  think  actively  upon  a  subject  and  becomes  men- 
tally listless — still  the  brain  can  never,  under  such  circum- 
stances, rest  properly,  and  sleep  finally  becomes  imperative. 

Vascular  Phenomena  of  Sleep. — Coma  is  analogous  to 
sleep  in  that  consciousness  is  lost ;  but  in  this  case  the  brain 
is  congested  and  the  condition  is  unnatural.  It  was  long 
supposed  that  this  was  the  vascular  condition  during  natural 
sleep,  but  application  of  the  physiological  principles  prevail- 
ing in  other  parts  of  the  body  would  rather  presuppose  a 
condition  of  cerebral  anemia;  for  the  brain  receives  blood 
for  two  purposes — first,  to  supply  nutrition  to  the  nervous 
substance,  and  second,  to  bring  supplies  which,  by  the  ac- 
tion of  the  brain  cells,  may  be  converted  into  nerve  force — 
and  during  sleep  only  the  first  of  these  purposes  is  to  be 
served.  This  is  true  in  case  of  glands,  muscles,  etc.,  during 
their  intervals  of  repose.  As  a  matter  of  fact,  the  cerebral 
vessels  are  contracted  and  there  is  much  less  blood  in  the 
brain  during  sleep  than  during  consciousness. 

Dreams. — In  explanation  of  the  phenomena  of  dreams  and 
somnambulism,  it  is  said  that  what  we  call  sleep  may  occur  in 
one  part  of  the  brain  and  not  in  another,  or  in  different  de- 
grees in  different  parts  of  the  nervous  centers.  "In  the 
former  case  [dreams]  the  cerebrum  is  still  partially  active ; 
but  the  mind  products  of  its  action  are  no  longer  corrected 
by  the  reception,  on  the  part  of  the  sleeping  sensorium,  of 
impressions  of  objects  belonging  to  the  outer  world ;  neither 


THE  SYMPATHETIC  SYSTEM  303 

can  the  cerebrum,  in  this  half-awake  condition,  act  on  the 
centers  of  reflex  action  of  the  voluntary  muscles,  so  as  to 
cause  the  latter  to  contract — a  fact  within  the  painful  experi- 
ence of  all  who  have  suffered  from  nightmare.  In  somnam- 
bulism the  cerebrum  is  capable  of  exciting  that  train  of  re- 
flex nervous  action  which  is  necessary  for  progression,  while 
the  nerve  center  of  muscular  sense  (in  the  cerebellum?)  is 
presumably  fully  awake;  but  the  sensorium  is  still  asleep, 
and  impressions  made  on  it  are  not  sufficiently  felt  to  rouse 
the  cerebrum  to  a  comparison  of  the  difference  between 
mere  ideas  or  memories  and  sensations  derived  from  exter- 
nal objects"  (Kirkes). 

Relation  Between  the  Cerebro-spinal  and  Sympathetic 
Systems. — A  brief  resume  may  help  to  clarify  the  association 
between  the  two  systems. 

1.  Anatomically. — The  two  are  developed  from  the  same 
embryological  tissue ;  the  vaso-motor  sympathetic  fibers  obey 
centers  in  the  medulla  and  cord,  and  must,  therefore,  be  con- 
nected with  those  centers  either  directly  or  indirectly ;  char- 
acteristic small  medullated  fibers  pass  at  intervals  from  the 
cord  through  the  roots  into  the  sympathetic  ganglia;  they 
send  fibers  each  to  the  trunks  of  the  other  to  be  distributed 
directly,  or  to  form  plexuses  and  then  be  distributed  to- 
gether; their  fibers  are  found  together  in  all  organs  which 
receive  cerebro-spinal  nerves  (unless  they  be  non- vascular)  ; 
in  some  of  these  organs  just  named  the  sympathetic  fibers 
are  there  only  as  vaso-motor  nerves,  while  in  others,  as 
glandular  structures  like  the  liver  and  salivary  glands,  sym- 
pathetic fibers  are  distributed  to  the  gland  cells  themselves, 
and  both  have  a  definite  but  associated  influence  on  secretion. 

2.  Physiologically. — The  physiological  relation  is  best  indi- 
cated by  examples.    A  great  many,  if  not  all,  the  sympathetic 
ganglia  seem  to  receive  their  power  to  generate  nerve  force 
from  the  cerebro-spinal  system ;  there  can  be  no  proper  nu- 
trition of  the  parts  animated  by  cerebro-spinal  fibers  with- 
out the  associated  aqtion  of  vaso-motor  sympathetic  fibers — 


3O4  THE  NERVOUS  SYSTEM 

not  even  of  the  nerve  cells  and  fibers  themselves;  in  reflex 
action  the  afferent  impression  may  be  conveyed  by  a  cerebro- 
spinal  fiber  and  reflected  through  a  sympathetic,  or  vice 
versa;  when  one  hand  is  thrust  into  hot  or  cold  water  the 
temperature  of  the  opposite  hand  may  be  raised  or  lowered, 
impressions  having  been  carried  to  the  center  by  cerebro- 
spinal  and  reflected  by  sympathetic  fibers,  not  only  to  the 
immersed  hand,  but  to  the  other  as  well ;  food  is  taken  into 
the  mouth,  impressions  are  carried  by  nerves  of  common 
sensation  to  the  brain  and  are  reflected  through  the  sympa- 
thetic system,  an  increased  amount  of  blood  is  thereby  sent 
to  the  salivary  glands  and  an  increased  secretion  supervenes ; 
one  smells  savory  articles  and  the  mouth  waters,  etc. 

Examples  could  be  multiplied  ad  infinitum  to  establish 
the  cooperation  existing  between  the  two  systems.  What 
has  been  incidentally  and  indirectly  said  on  this  point  in  con- 
sidering secretion,  digestion,  circulation,  respiration,  etc., 
serves  to  emphasize  their  connection. 


CHAPTER  XII. 
THE  SENSES. 

IT  is  evident  from  preceding  remarks  that  it  is  through  the 
intervention  of  the  nervous  system  that  we  have  a  "sense"  of 
existence,  of  the  existence  and  condition  of  different  parts  of 
our  bodies  and  of  our  relations  to  the  external  world.  The 
knowledge  we  thus  obtain  is  based  upon  sensations  of  various 
kinds,  all  of  which  are  carried  to  the  centers  by  afferent 
fibers.  Such  sensations  may  be  what  are  termed  (A)  Com- 
mon, or  (B)  Special,  including  (i)  Touch,  (2)  Smell,  (3) 
'Sight,  (4)  Taste  f  (5)  Hearing.  It  is  to  be  remembered  that 
the  seat  of  sensation  is  in  the  brain,  and  not  in  any  organ 
which  primarily  receives  or  conveys  the  impression.  We  do 
not  in  reality  see  with  the  eye  or  hear  with  the  ear ;  these  are 
only  complex  organs  so  arranged  that  rays  of  light  or  sound 
waves  produce  upon  them  such  impressions  as,  when  trans- 
mitted to  the  sensorium,  will  give  rise  to  the  sensations  of 
sight  or  hearing. 

(A)  COMMON  SENSATIONS. 

As  regards  the  uses  of  the  fibers  conveying  impressions 
which  result  in  these  sensations,  they  (unless  it  be  those  con- 
cerned with  tactile  impressions)  are  distinct  from  those  of 
special  sense.  That  is  to  say,  the  fibers  of  the  olfactory, 
optic,  gustatory  and  auditory  nerves  do  not  convey  general 
impressions;  but  it  is  almost  certain  that  fibers  conveying 
tactile  impressions  convey  also  painful  impressions — and  the 
sensation  of  pain  is  taken  as  typical  of  common  sensations. 
It  is  known  that  very  painful  impressions  sometimes  over- 
come tactile  sensibility,  and  that  very  frequently  tactile  sen- 

20  305 


306  THE  SENSES 

sibility  remains  in  parts  which  receive  no  painful  impres- 
sions, as  e.  g.,  under  anesthesia  by  cocain ;  but  it  may  be  that 
the  power  in  the  same  fiber  to  convey,  in  the  first  case  tactile, 
and  in  the  second  painful  impressions  is  destroyed  without 
destroying  its  power  to  convey  the  other. 

The  varieties  of  common  sensation  are  too  numerous  to 
even  mention.  Thirst,  hunger,  fatigue,  discomfort,  satiety, 
etc.,  are  everyday  examples,  as  are  also  the  desire  to  urin- 
ate or  defecate.  Numerous  subdivisions  of  the  sensation  of 
pain  might  be  mentioned,  such  as  itching,  burning,  aching, 
etc.  The  so-called  muscular  sense — by  which  we  become 
aware  of  the  condition,  relation,  coordination  and  degree  of 
activity  or  repose  of  the  muscles — will  be  considered  as  be- 
longing here. 

(B)  SPECIAL  SENSATIONS, 
i.  The  Sense  of  Touch. 

The  sense  of  touch  is  closely  related  to  common  sensation. 
Its  distribution  over  the  body  is  as  uniform  as  that  of  com- 
mon sensation,  but  it  is  most  highly  developed  in  those  parts 
where  general  sensibility  is  most  marked  (as  in  the  skin), 
and  attains  its  highest  degree  of  perfection  only  in  those 
situations  in  which  tactile  corpuscles  exist,  for  example,  on 
the  palmar  surfaces  of  the  tips  of  the  fingers.  The  teeth, 
hair,  nails,  etc.,  are  rather  surprisingly  endowed  with  tactile 
sensibility.  Leaving  pain  and  the  muscular  sense  as  part  of 
general  sensibility,  the  sense  of  touch  may  be  considered 
under  two  heads — (a)  Tactile  Sensibility  proper  and  (b) 
Temperature. 

(a)  Tactile  sensibility  proper  is  most  marked  where  the 
epidermis  over  the  papillae  is  thin.  When  the  epidermis  is 
removed  and  the  cutis  is  touched  there  is  pain  instead.  Tac- 
tile sensibility  is  much  decreased  where  the  epidermis  is 
thickened,  as  over  the  heel.  The  terminal  tactile  organs 


THE  SENSE  OF  SMELL.  307 

have  been  described  in  connection  with  afferent  nerves. 
They  are  chiefly  the  end  bulbs  of  Krause  and  the  tactile 
corpuscles  of  Meissner.  (See  Figs.  71  and  72).  Besides, 
tactile  impressions  are  received  by  the  free  extremities  of 
afferent  nerves  situated  over  the  body  at  large.  Numbness 
from  cold  is  due  to  interference  with  cutaneous  circulation 
— upon  which  the  sense  of  touch  is  directly  dependent.  It  is 
almost  impossible  to  distinguish  mere  touch  from  pressure. 

Acuteness. — How  the  sense  of  touch  is  capable  of  devel- 
opment by  practice  is  well  illustrated  in  the  case  of  many 
blind  persons.  They  learn  to  read  with  comparative  facility 
by  passing  the  hand  over  raised  letters ;  or  they  frequently 
make  the  sense  of  touch  take  the  place  of  the  lost  sense  in 
other  almost  incredible  ways.  The  acuteness  of  this  sense 
in  different  portions  of  the  body  has  been  made  the  subject 
of  observation  by  touching  two  different  parts  in  the  same 
region  with  finely  pointed  instruments  and  noting  how  near 
the  points  can  be  brought  together  and  still  be  recognized  as 
two.  This  distance  is  found  to  vary  from  ^4  inch  on  the  tip 
of  the  tongue  to  2^2  inches  in  the  dorsal  region. 

(b)  It  is  not  improbable  that  there  are  special  nerve  end- 
ings concerned  in  the  reception  of  temperature  impressions, 
though  this  has  not  been  definitely  proven.  Decisions  as  to 
temperature  are  only  relative;  the  surface  temperature  of 
the  part  upon  which  the  impression  is  made  is  the  standard, 
and  one  can  only  tell  absolutely  whether  the  object  is  hotter 
or  colder  than  the  skin,  and,  within  certain  limits,  approxi- 
mate how  much  hotter  or  colder.  The  delicacy  of  the  tem- 
perature sense  agrees  with  that  of  touch  as  regards  the  thick- 
ness or  absence  of  the  epidermis. 

2.  The  Sense  of  Smell. 

Regarding  the  mechanism  of  olfaction  it  is  found  that  one 
of  the  .first  conditions  necessary  is  the  presence  of  particular 
cells.  Between  the  epithelial  cells  of  the  mucous  membrane 


308  THE  SENSES 

to  which  the  olfactory  fibers  are  distributed  are  delicate 
spindle-shaped  cells  known  as  olfactory  cells,  and  to  them 
pass  the  terminal  filaments  from  the  olfactory  bulbs.  These 
cells  are  stimulated  by  contact  with  odorous  substances,  and 
from  them  go,  by  way  of  the  nerve  fibers,  impressions  which 
are  recognized  as  odors  of  different  kinds.  The  olfactory 
fibers  are  the  only  ones  which  will  convey  such  impressions. 
True,  the  same  substance  may,  at  the  same  time,  excite  other 
sensations,  as  of  pain  or  taste,  but  the  impressions  giving 
rise  to  these  latter  sensations  are  conveyed  by  different  fibers 
altogether.  The  substances  which  excite  olfaction  must 
come  in  actual  contact  with  the  nerve  terminals  and  to  do 
this  must  be  dissolved  in  the  mucus  of  the  nasal  mucous 
membrane;  hence  dryness  of  the  nasal  cavities  (as  in  the 
first  stage  of  nasal  catarrh)  interferes  with  olfaction.  It  is 
also  said  that  odorous  substances  introduced  in  solution  into 
the  nasal  cavities  will  not  excite  the  sense  of  smell,  but  that 
they  must  be  introduced  by  a  current  of  air. 

Whether  an  odor  is  pleasant  or  unpleasant  is  largely  a 
relative  matter ;  odors  most  disgusting  to  some  animals  are 
not  offensive  to  others.  This  same  difference  may  also 
hold  good  among  different  men.  Impairment  of  the  sense 
of  taste,  for  some  reason,  follows  a  loss  of  the  sense 
of  smell. 

3.  The  Sense  of  Sight. 

It  is  not  intended  to  go  into  a  detailed  consideration  of  the 
sense  of  sight,  but  some  remarks  on  the  normal  eye  and  its 
action  are  in  order. 

Protection  of  the  Ball. — The  orbital  cavity  has  a  pyra- 
midal shape  with  its  base  forward.  It  contains  the  eye-ball, 
its  muscles,  some  adipose  tissue  and  most  of  the  lachrymal 
apparatus.  Above  the  orbit,  the  eye-brows  prevent  a  flow  of 
perspiration  from  the  forehead  on  to  the  lid,  and  also  shade 
the  eye  to  some  extent.  The  lids,  when  closed,  entirely  ob- 


MOVEMENTS  OF  THE  BALL  309 

scure  the  balls  and  protect  them  in  front.  On  their  free  bor- 
ders are  rows  of  hairs  (eye-lashes)  curling  away  from  the 
globe  and  shading  and  protecting  it  from  dust.  The  lids 
are  closed  by  the  orbicular es  palp ebr arum  and  opened  by  the 
levatores  palpebrarum  superiores.  In  the  ordinary  closing 
of  the  lids  only  the  upper  one  is  moved,  but  the  lower  one  is 
raised  in  forcible  contraction  of  the  orbicularis.  Interven- 
tion of  the  will  is  not  necessary  to  the  action  of  these  mus- 
cles, though  they  are  striated.  Except  during  fatigue,  the 
eyes  are  kept  open  involuntarily,  but  when  the  cornea  is 
touched  no  effort  of  the  will  can  prevent  contraction  of  the 
orbicularis  palpebrarum.  During  sleep  the  globes  are 
rotated  upward. 

The  Lachrymal  Apparatus. — This  consists  of  the  lachry- 
mal glands,  canal,  duct  and  sac,  and  the  nasal  duct.  The 
secretion  of  the  lachrymal  gland  keeps  the  cornea  and  con- 
junctiva constantly  bathed  in  a  thin  fluid.  It  is  situated  in 
the  orbital  cavity  at  its  upper  and  outer  portion.  Its  secre- 
tion is  discharged  upon  the  conjunctiva  by  several  little 
ducts.  The  excess  of  secretion  is  carried  into  the  nose 
through  the  nasal  duct.  Near  the  inner  canthus  is  a  small 
opening  in  each  lid;  these  openings  are  the  orifices  of  the 
lachrymal  canals,  which  canals  join  at  the  inner  angle  of  the 
eye  to  form  the  lachrymal  sac ;  the  sac  is  continued  below  as 
the  nasal  duct,  opening  into  the  inferior  meatus  of  the  nose. 
The  secretion  of  tears  is  much  diminished  during  sleep.  The 
influence  of  the  nervous  system  on  lachrymal  secretion  is 
well  known.  Emotional  disturbances  operate  through  the 
sympathetic  to  increase  the  flow.  Irritation  of  the  mucous 
membrane  of  the  nose  or  eye  is  followed  by  a  like  result. 

Movements  of  the  Ball. — The  capsule  of  Tenon,  a  fibrous 
membrane  outside  the  sclerotic,  holds  the  ball  loosely  in 
place.  A  small  amount  of  adipose  tissue  behind  the  globe  is 
never  absent.  Movements  of  the  ball  are  effected  through 
the  action  of  the  internal  and  external  recti,  the  superior 
and  inferior  recti,  and  the  superior  and  inferior  oblique; 


3IO  THE  SENSES 

of  these,  all  but  the  two  last  named  arise  from  the  apex  of 
the  orbital  cavity.  The  recti  are  inserted  into  the  sclerotic 
just  back  of  the  cornea.  The  superior  oblique  runs  along 
the  inner  aspect  of  the  orbital  cavity  to  a  point  near  the 
supero-internal  angle;  here  it  becomes  tendinous,  passes 
through  a  fibro-cartilaginous  ring,  and  then  turns  backward 
and  outward  to  be  inserted  into  the  sclerotic  between  the 
superior  and  external  recti  just  behind  the  center  of  the 
globe.  The  inferior  oblique  arises  just  within  the  orbital 
cavity  near  the  anterior  inferior  angle,  and  passes  around  the 


FIG.  88. — Muscles  of  the  eye  and  tendon  or  ligament  of  Zinn. 

i,  tendon  of  Zinn;  2,  external  rectus  divided;  3,  internal  rectus;  4,  inferior 
rectus;  5,  superior  rectus;  6,  superior  oblique;  7,  pulley  for  superior  oblique; 
8,  inferior  oblique;  9,  levator  palpebrse  superioris;  10,  10,  its  anterior  expansion; 
n,  optic  nerve.  (Sappey.) 


anterior  part  of  the  globe  to  be  inserted  in  the  sclerotic  just 
below  the  superior  oblique. 

The  effect  these  muscles  have  upon  the  movements  of  the 
ball  is  indicated  by  their  origin  and  attachment.  The  exter- 
nal and  internal  recti  rotate  it  outward  and  inward,  the  su- 
perior and  inferior  recti  upward  and  downward.  The  su- 


ANATOMY  OF  THE  BALL  3!  I 

perior  and  inferior  oblique  antagonize  each  other.  The 
former  rotates  the  globe  so  that  the  pupil  is  directed  out- 
ward and  downward ;  the  latter  so  that  it  looks  outward  and 
upward.  The  associated  action  of  all  these  muscles  can  pro- 
duce almost  any  variety  of  movements,  and  no  effort  of  the 
will  is  necessary  to  properly  associate  them  when  it  is  desired 
to  direct  the  line  of  vision  toward  a  certain  object.  For  in- 
stance, when  it  is  desired  to  look  at  an  object  on  the  right  it 
takes  no  distinct  voluntary  effort  to  contract  the  external 
rectus  of  the  right  eye  and  the  internal  rectus  of  the  left.  It 
will  be  seen  later  that  vision  for  the  two  eyes  is  normal  only 
when  impressions  are  made  upon  exactly  corresponding 
parts  of  the  two  retinae,  so  that  they  may  act  as  a  single  or- 
gan ;  and  for  this  to  be  done  not  always  the  same  movements 
are  called  for  in  both  balls. 

Anatomy  of  the  Ball. — The  eye-ball  is  a  globular  body 
consisting  of  several  coats  enclosing  refracting  media.  Of 
these  coats  the  external  is  the  sclerotic,  dense  and  fibrous, 
covering  the  posterior  five-sixths  of  the  organ  and  continu- 
ous with  the  cornea,  which  covers  the  anterior  one-sixth.  It 
is  not  well  supplied  with  blood-vessels.  The  cornea  is  trans- 
parent, and  upon  its  external  surface  are  several  layers  of 
delicate  nucleated  epithelium;  underneath  this  layer  of  cells 
is  a  thin  membrane,  the  anterior  elastic  lamella,  which  is  a 
continuation  of  the  conjunctiva.  The  substance  proper  of 
the  cornea  is  composed  of  pale  interlacing  fibers  among 
which  are  connective  tissue  corpuscles  and  quite  a  quantity 
of  fluid.  These  fibers  are  continuous  from  the  sclerotic,  but 
they  lose  their  opacity  at  the  corneo-sclerotic  margin.  On 
the  posterior  surface  of  the  cornea  is  the  transparent  elastic 
membrane  of  Descemet,  a  part  of  which,  at  the  circumfer- 
ence of  the  iris,  passes  into  the  ciliary  muscle.  The  cornea 
is  very  sensitive,  but  contains  no  blood-vessels. 

Next  inside  the  sclerotic  is  the  choroid  coat  of  the  eye.  It 
does  not  lie  under  the  cornea,  but  is  confined  to  the  sclerotic 
area  of  the  ball  Behind  the  optic  nerve  penetrates  it,  and  in 


3I2 


THE  SENSES 


front  it  is  connected  with  the  iris.  The  choroid  is  very  vas- 
cular. Its  color  is  dark  brown  on  account  of  the  abundance 
of  pigment  in  the  cells  on  the  inner  surface  of  the  mem- 
brane. Anteriorly  the  choroid  is  folded  in  upon  itself  to 
form  the  ciliary  processes,  which  project  inward  around  the 
margin  of  the  crystalline  lens. 

The  ciliary  muscle  is  important  in  accommodation.    It  is 


FIG.  89. — Diagram  of  a  vertical  section  of  the  eye.     (From  Yeo 
after  H olden.} 

i,  anterior  chamber  filled  with  aqueous  humor;  2,  posterior  chamber;  3, 
canal  of  Petit;  a,  hvaloid  membrane;  b,  retina  (dotted  line);  c,  choroid  coat 
(black  line);  d,  sclerotic  coat;  e,  cornea;  f,  iris;  g,  ciliary  processes;  h,  canal  of 
Schlemm  or  Fontana;  i,  ciliary  muscle. 

in  the  shape  of  a  muscular  ring  surrounding  the  margin  of 
the  choroid  just  outside  the  ciliary  processes.  In  front  it  is 
attached  to  the  line  of  junction  of  the  cornea  and  sclerotic 
and  to  the  ligament  on  the  anterior  surface  of  the  iris ;  be- 
hind it  is  lost  in  the  substance  of  the  choroid.  Its  contrac- 
tion, therefore,  compresses  the  vitreous  humor  and  relaxes 
the  suspensory  ligament  of  the  lens.  The  iris  is  a  circular 
veil  hanging  in  front  of  the  lens.  It  presents  a  perforation  a 


ANATOMY  OF  THE  BALL  313 

little  to  the  nasal  side  of  its  center,  the  pupil.  It  is  attached 
to  the  corneo-sclerotic  line.  It  contains  circular  and  radi- 
ating fibers.  The  iris  divides  the  space  between  the  cornea 
and  lens  into  two  chambers,  anterior  and  posterior — the  lat- 
ter of  which  is  very  small.  The  "color  of  the  eyes"  depends 
on  the  color  of  the  anterior  surface  of  the  iris ;  its  posterior 
surface  has  a  constant  dark  purple  hue.  The  size  of  the  pu- 
pil is  subject  to  variations  to  be  noted  later. 

Inside  the  choroid  is  the  retina,  which  is  that  part  of  the 
eye  capable  of  receiving  impressions  of  sight.  Anteriorly  it 
reaches  nearly  to  the  ciliary  processes.  Externally  it  is  in 
contact  with  the  choroid,  and  internally  with  the  hyaloid 
membrane  of  the  vitreous  humor.  It  is  penetrated  by  the 
optic  nerve  a  little  within  and  below  the  center  of  the  pos- 
terior hemisphere.  Just  external  to  the  point  of  entrance  of 
the  nerve  is  the  macula  lutea,  a  small  yellow  area  in  the  cen- 
ter of  which  is  the  fovea  centralis;  this  last  is  exactly  in  the 
axis  of  distinct  vision.  Nine  layers  of  cells  are  usually  de- 
scribed as  composing  the  retina.  From  without  inward  they 
are  (i)  the  pigment  layer,  (2)  rods  and  cones,  (3-6)  the 
four  granular  layers,  (7)  nerve  cells,  (8)  expansion  of  fibers 
of  the  optic  nerve,  (9)  the  limitary  membrane.  Of  these,  the 
most  important  is  the  layer  of  rods  and  cones.  The  rods,  or 
cylinders,  extend  through  the  thickness  of  the  membrane  and 
have  between  them,  at  intervals,  flask-shaped  bodies,  the 
cones.  At  the  macula  lutea  only  the  cones  exist.  Elsewhere 
the  rods  are  more  abundant  than  the  cones. .  The  length  of 
the  cones  is  about  half  that  of  the  rods,  and  they  occupy  the 
inner  aspect  of  the  membrane.  The  layer  of  nerve  cells  pre- 
sents cells  communicating  on  the  one  hand  with  the  rods  and 
cones  and  on  the  other  with  fibers  of  the  optic  nerve.  The 
rods  and  cones  are  the  only  parts  of  the  retina  possessing 
special  sensibility,  impressions  being  conveyed  from  them 
to  the  brain  by  the  optic  nerve.  The  fibers  of  the  second 
nerve,  composing  one  layer,  are  pale  and  transparent.  The 
blood  supply  of  the  retina  is  from  the  arteria  centralis 


314  THE  SENSES 

retinae,  which  enters  the  optic  nerve  just  before  it  expands, 
and,  running  in  its  substance,  is  distributed  as  far  as  the 
ciliary  processes  anteriorly. 

The  Crystalline  Lens  is  a  biconvex  transparent  body  situ- 
ated just  behind  the  iris.  Its  function  is  to  refract  the  rays 
of  light,  and  its  action  in  this  respect  is  similar  to  such 
lenses  in  optical  instruments.  It  is  held  in  place  by  the  sus- 
pensory ligament.  Its  anterior  convexity  is  more  marked 
than  its  posterior.  It  is  enveloped  by  a  thin  transparent  cap- 
sule. 

The  Suspensory  Ligament  is  a  continuation  of  the  an- 
terior layer  of  the  hyaloid  membrane  of  the  vitreous  humor. 
When  this  layer  reaches  the  edge  of  the  lens  (coming  for- 
ward) it  divides  into  two  parts,  one  passing  in  front  of  and 
the  other  behind  that  body ;  the  divisions  are  continuous  re- 
spectively with  the  anterior  and  posterior  portions  of  the  cap- 
sule of  the  lens.  The  ligament  supports  the  lens. 

The  Aqueous  Humor  is  behind  the  cornea  and  in  front  of 
the  lens  and  suspensory  ligament.  The  iris  has  been  said  to 
separate  this  cavity  into  anterior  and  posterior  chambers 
communicating  through  the  pupillary  opening.  The  aqueous 
humor  is  colorless  and  perfectly  transparent.  It  serves  to 
refract  the  rays  of  light,  having  for  that  purpose  the  same 
index  as  the  cornea. 

The  Vitreous  Humor  occupies  about  the  posterior  two- 
thirds  of  the  globe,  and  is  back  of  the  lens  and  suspensory 
ligament  surrounded  by  the  delicate  hyaloid  membrane.  It 
is  of  a  gelatinous  consistence,  and  is  divided  into  numerous 
compartments  by  very  delicate  membranes  radiating  from 
the  point  of  entrance  of  the  optic  nerve.  It  is  a  transparent 
refracting  medium. 

Ocular  Refraction. — In  order  for  the  image  of  an  object 
to  be  distinct  the  rays  passing  from  it  must  fall  on  a  single 
portion  of  the  retina,  viz.,  the  fovea  centralis.  The  sensi- 
bility of  the  retina  to  light  decreases  in  passing  away  from 
the  fovea.  All  rays  would  not  meet  on  the  retina  unless  they 


ACCOMMODATION  315 

were  refracted;  and  for  this  purpose  there  are  the  cornea, 
the  aqueous  humor,  the  lens  and  the  vitreous  humor.  The 
surfaces  of  the  cornea  and  lens  are  the  most  important  of 
these.  Since  the  two  surfaces  of  the  cornea  are  parallel,  the 
external  surface  alone  is  concerned  in  refraction.  The  cen- 
ter of  distinct  vision  (fovea)  is  in  the  axis  of  the  lens 
precisely  in  the  plane  upon  which  the  rays  of  light  are 
brought  to  a  focus  by  the  refracting  media.  Refraction  by 
the  cornea  alone  would  focus  the  rays  behind  the  retina; 
hence  the  necessity  of  convex  lenses  before  the  eye  after  op- 
erations for  cataract.  Rays  leaving  the  cornea  are  refracted 
by  the  anterior  surface  of  the  lens,  by  its  substance  to  a  cer- 
tain extent,  and  again  by  its  posterior  surface,  the  normal 
mechanism  being  such  that  all  rays  are  focused  on  the  fovea. 
The  rays  cross  each  other  after  refraction,  and  the  image  is 
inverted,  but  the  brain  takes  no  notice  of  this  fact,  and  ob- 
jects are  seen  in  their  natural  positions. 

Accommodation. — Accommodation  means  a  change  in  the 
convexity  of  the  lens,  whereby  images  are  focused  on  the 
retina,  whether  the  object  be  far  away  from  or  near  the  eye. 
Rays  of  light  from  distant  objects  strike  the  eye  practically 
parallel,  and  we  may  assume  that  there  is  a  certain  "passive" 
condition  of  the  refracting  media  which  will  bring  such  rays 
to  a  focus  at  the  proper  point.  But  when  the  object  ob- 
served is  near  the  eye  a  change  in  the  arrangement  of  the 
media,  or  of  the  convexity  of  their  surfaces,  is  necessary  to 
prevent  the  focusing  of  the  rays  behind  the  retina.  The  de- 
sired end  is  accomplished  by  increasing  the  convexity  of  the 
lens.  When  the  ciliary  muscle  is  "passive"  the  capsule  com- 
presses the  lens,  decreasing  its  convexity  to  a  minimum; 
from  the  attachments  of  this  muscle,  already  noted,  its  con- 
traction is  attended  by  a  relaxation  of  the  suspensory  liga- 
ment, which  in  turn  relieves  in  some  degree  the  compression 
of  the  capsule  upon  the  lens  and  allows  its  antero-posterior 
diameter  to  increase ;  the  result  is  increased  convexity  of  the 
lens. 


THE  SENSES 

When  distant  objects  are  looked  at  the  lens  becomes  flatter 
as  a  result  of  the  contraction  of  the  suspensory  ligament, 
which  contraction  is  a  consequence  of  the  relaxation  of  the 
ciliary  muscle.  Accommodation  for  distant  objects  seems 
a  passive  process  entirely. 

The  ciliary  muscle  is  the  "muscle  of  accommodation." 

The  contraction  of  the  pupil  for  near  objects  is  not,  prop- 
erly speaking,  a  part  of  accommodation. 

Then,  granting  special  sensibility  to  the  retina  and  optic 
nerve  the  formation  and  appreciation  of  an  image  is  simple. 
Rays  of  light  having  passed  through  the  cornea  and  aqueous 
humor  are  admitted  by  the  pupil  to  pass  through  the  lens  and 
vitreous  humor.  By  all  these  objects  they  are  refracted  so 
that  they  cross  each  other  and  fall  upon  the  retina,  producing 
an  inverted  image  there.  The  size  of  the  pupil,  other  things 
being  equal,  is  regulated  by  the  intensity  of  the  light,  the 
opening  being  contracted  to  admit  less  when  the  light  is 
strong. 

Myopia,  Hyperopia  and  Presbyopia. — Sometimes  the  an- 
tero-posterior  diameter  of  the  eye-ball  is  too  long  and  the 
rays  of  light  are  brought  to  a  focus  in  front  of  the  retina. 
Such  a  condition  is  known  as  myopia;  the  person  will  be 
near-sighted.  He  brings  objects  near  his  eyes  so  that  the 
rays  may  have  a  greater  divergence  and  thus  be  focused  far- 
ther back.  Or  the  rays  may  be  scattered  by  placing  concave 
lenses  before  the  eyes.  Sometimes,  too,  the  antero-posterior 
diameter  may  be  too  short  and  the  rays  come  to  a  focus  be- 
hind the  retina.  Such  a  condition  is  known  as  hyperopia; 
the  person  will  be  far-sighted.  He  holds  objects  far  away 
from  his  eyes  that  the  rays  from  them  may  strike  the  ball 
with  less  divergence  and  thus  be  focused  farther  forward. 
Or  the  same  end  may  be  accomplished  by  placing  convex 
lenses  before  the  eyes.  In  old  age  the  lens  becomes  flattened 
and  accommodates  itself  less  easily.  This  tends  to  focus  light 
behind  the  retina  and  objects  have  to  be  held  far  away  from 


THE  SENSE  OF  TASTE  317 

the  eye.  This  is  known  as  presbyopia.  Its  remedy  is  the 
same  as  that  for  hyperopia. 

Reaction  to  Light. — Regarding  the  reaction  of  the  pupil 
to  light,  it  is  evident  that  this  is  mainly  a  reflex  nervous  phe- 
nomenon, though  direct  light  will  cause  the  muscular  tissue 
of  the  iris  to  contract.  The  direct  influence  of  the  third 
nerve  on  the  action  of  the  iris  has  been  referred  to  under  a 
consideration  of  that  nerve.  Reflexly,  the  pupil  is  con- 
tracted by  light  by  the  conveyance  of  an  impression  to  the 
brain  through  the  optic  fibers,  a  message  is  sent  to  the  pro- 
per center,  and  a  stimulus  is  reflected  through  the  third  nerve 
to  the  sphincter  of  the  iris  causing  it  to  contract.  When  the 
optic  nerve  is  cut  the  circuit  is  broken,  and  movements  of 
the  iris  do  not  occur  from  the  admission  of  light.  Practic- 
ally, then,  when  much  or  little  light  reaches  the  retina  the  pu- 
pil contracts  or  dilates,  as  the  case  may  be,  in  an  effort  to 
keep  the  amount  constant. 

Binocular  Vision. — It  is  evident  that  when  a  person  looks 
at  an  object  two  images  are  formed — one  on  each  retina — 
but  they  are  combined  in  his  consciousness  and  he  sees  but 
one  object.  If  one  of  the  balls  be  thrown  out  of  the  proper 
axis,  by  pressure,  e.  g.,  objects  appear  double.  The  same  is 
true  in  strabismus,  at  least  until  the  person  has  grown  ac- 
customed to  the  defect.  In  normal  vision  the  rays  from 
an  object  are  formed  on  the  fovea  centralis  of  each  eye — that 
is,  upon  corresponding  points  which  are,  for  each,  the  centers 
of  distinct  vision. 

4.  The  Sense  of  Taste. 

In  order  that  gustatory  sensation  may  be  exercised  it  is 
necessary  (i)  that  there  be  specially  endowed  nerves  and 
nerve  centers;  (2)  that  the  nerve  terminals  be  excited  by 
sapid  (tastable)  materials;  (3)  that  these  substances  be  in 
solution.  It  has  already  been  seen  that  the  special  nerves  of 
taste  are  (a)  the  chorda  tympani  distributed  to  the  anterior 


31.8  THE  SENSES 

two-thirds  of  the  tongue,  and  (b)  the  glosso-pharyngeal  to 
the  posterior  third  of  that  organ.  It  is  probable  that  only  the 
dorsum  of  the  tongue,  the  lateral  parts  of  the  soft  palate,  the 
uvula  and  the  upper  pharynx  are  concerned  in  gustation.  On 
the  tongue  are  found  special  papillae,  ( i )  the  circumvallate, 
large  and  few  in  number,  near  the  base  of  the  organ,  and  (2) 
the  fungiform,  about  200  in  number,  over  the  remaining  area. 
The  circumvallate  and  some  of  the  fungiform  papillae  contain 
taste  beakers,  true  gustatory  organs.  They  are  ovoid  col- 
lections of  cells  beneath  the  epithelial  covering  of  the,  mu- 
cous membrane.  Sapid  substances  enter  these  beakers  in  so- 
lution and  come  in  contact  with  the  taste  cells,  which  are 
connected  with  the  filaments  of  the  gustatory  nerves.  Thus 
are  produced  specific  impressions  which  are  conveyed  to  the 
gustatory  center,  and  the  sense  of  taste  is  excited.  The  lim- 
ited distribution  of  the  taste  beakers  makes  it  impossible  that 
they  should  be  the  only  organs  capable  of  receiving  special 
gustatory  impressions.  The  taste  center  has  been  indefi- 
nitely located  in  the  uncinate  gyrus  near  the  olfactory  center. 

Since  it  is  necessary  to  the  tasting  of  substances  that  they 
come  in  actual  contact  with  the  taste  organs,  and  since  to  do 
so  they  must  be  in  solution,  it  follows  that  dryness  of  the 
mouth  interferes  with,  or  abolishes,  this  sense. 

The  most  marked  tastes  are  the  sweet,  bitter,  saline,  and 
alkaline.  The  more  delicate  flavors  involve  also  the  special 
sense  of  smell,  and  it  has  been  seen  that  dissociation  of  the 
two  kinds  of  impressions  is  often  impossible.  Taste  is  also 
subject  to  variations  by  reason  of  education,  age,  association, 
caprice,  etc.  Bitters  are  most  easily  appreciated  at  the  back, 
salts  and  sweets  at  the  tip,  and  acids  at  the  sides  at  the 
tongue. 

5.  The  Sense  of  Hearing. 

The  ear  consists  of  a  complicated  apparatus  for  the  pur- 
pose of  the  reception  of  special  impressions  which  are  appre- 
ciated by  the  brain  as  sounds.  Anatomically  it  consists  of 


THE  EXTERNAL  EAR 


319 


the  external,  the  middle  and  the  internal  ear ;  the  last  con- 
tains the  essentials  of  the  auditory  apparatus,  the  external 
and  middle  divisions  serving  only  to  concentrate  the  sound 
waves  upon  the  parts  of  the  internal. 
The  External  Ear. — This  consists  of  the  pinna  and  the 


FIG.  90. — Scheme  of  the  organ  of  hearing. 

AG,  external  auditory  meatus;  T,  tympanic  membrane;  K,  malleus  with  its 
head  (/i)»  short  process  (kf)  and  handle  (»t) ;  a,  incus,  its  short  process  (x) 
and  its  long  process  united  to  the  staples  (s)  by  means  of  the  Sylvian  ossicle 
(Z);  P,  middle  ear;  o,  fenestra  ovalis;  r,  fenestra  rotunda;  x,  beginning  of 
the  lamina  spiralis  of  the  cochlea;  pt,  scala  tympani,  and  vt,  scala  vestibuli;  V , 
vestibule;  S,  saccule;  U,  utricle;  H,  semicircular  canals;  TE,  Eustachian  tube. 
The  long  arrow  indicates  the  line  of  traction  of  the  tensor  tympani;  the  short 
curved  one,  that  of  the  stapedius.  (Landois.) 

external  auditory  canal.  The  pinna  is  the  external  visible 
portion,  and  consists  of  the  large  cavity,  the  concha,  into 
which  the  external  auditory  canal  opens  externally ; 
of  two  prominent  ridges  partly  surrounding  the  concha,  the 
helix  outside  and  the  antehelix  internal  to  this;  and  of  a 
fibro-cartilaginous  process  projecting  backward  in  front  of 
the  concha,  the  tragus.  The  external  auditory  canal  runs 


32O  THE  SENSES 

inward  and  slightly  forward  from  the  concha  to  terminate 
at  the  membrana  tympani,  or  drum.  Its  inner  part  is  in  the 
petrous  portion  of  the  temporal  bone;  its  external  part  is 
fibro-cartilaginous  in  structure.  The  whole  is  lined  by  in- 
tegument. 

The  Middle  Ear  (Tympanum). — This  is  a  cavity  at  the 
bottom  of  the  external  auditory  canal  in  the  petrous  portion 
of  the  temporal  bone,  containing  ossicles  for  the  conduction 
of  sound  waves  to  the  internal  ear.  The  cavity  communi- 
cates, through  the  Eustachian  tube,  with  the  pharynx,  and 
this  is  its  only  direct  connection  with  the  external  air,  though 
it  does  communicate  with  the  mastoid  air  cells.  It  is  lined  by 
mucous  membrane.  The  membrana  tympani,  separating  it 
from  the  external  auditory  canal,  is  fibrous  in  structure.  It 
is  lined  externally  by  skin  and  internally  by  mucous  mem- 
brane. 

The  three  ossicles  of  the  middle  ear  are  the  malleus,  incus 
and  stapes.  The  malleus,  shaped  like  a  hammer,  is  attached 
in  a  vertical  direction  to  the  upper  radius  of  the 
membrana  tympani,  and  articulates  by  its  head  with  the 
incus.  The  incus  has  the  shape  of  an  anvil ;  its  base  articu- 
lates with  the  malleus,  while  its  small  extremity  curves 
downward  to  articulate  with  the  neck  of  the  stapes.  The 
base  of  the  stapes  is  applied  to  the  membrane  covering  the 
fenestra  ovalis.  The  tensor  and  laxator  tympani  are  at- 
tached to  the  neck  of  the  malleus ;  the  stapedius  to  the  neck 
of  the  stapes.  These  bones  constitute  a  chain,  which  con- 
veys the  vibrations  of  the  membrana  tympani  to  the  fenestra 
ovalis. 

The  Internal  Ear  (Labyrinth). — This  consists  of  a  series 
of  cavities  in  the  petrous  portion  of  the  temporal  bone  lined 
by  a  peculiar  membrane.  When  the  bony  substance  sur- 
rounding these  cavities  is  carefully  removed  it  is  found 
that  that  portion  immediately  outside  them  is  harder  than 
the  adjacent  structure.  This  constitutes  the  bony  labyrinth, 
while  the  membrane  inside  the  bony  walls  is  the  membranous 
labyrinth. 


THE  BONY  LABYRINTH 


32I 


The  bony  labyrinth  consists  of  the  vestibule,  cochlea  and 
semicircular  canals.    The  vestibule  occupies  the  mid-portion 


FIG.  91. 

/,  Transverse  section  of  a  turn  of  the  cochlea;  II,  A,  ampulla  of  a  semicircu- 
lar canal  with  the  crista  acustica;  a,  auditory  cells;  p,  provided  with  a  fine  hair; 
T,  otoliths;  ///,  scheme  of  the  human  labyrinth;  IV,  scheme  of  a  bird's  laby- 
rinth; V,  scheme  of  a  fish's  labyrinth.  (Landois.) 

of  the  labyrinth,  and  is  that  part  with  which  the  middle  ear 
communicates  by  the  fenestra  ovalis;  it  communicates  also 

21 


THE  SENSES 

with  the  cochlea  and  semicircular  canals,  and  on  its  internal 
aspect  are  openings  for  the  entrance  of  some  of  the  branches 
of  the  auditory  nerve.  The  cochlea,  shaped  like  a  snail  shell, 
runs  off  from  the  front  of  the  vestibule,  winds  about  two 
and  a  half  times  around  a  cone-shaped  central  axis — the 
modiolus — and  ends  in  a  blind  apex.  The  canal  of  the  coch- 
lea is  partially  separated  into  two  compartments  by  a  bony 
plate,  the  lamina  spiralis. 

The  basilar  membrane  completes  the  septum  and  divides 
the  lumen  of  the  cochlea  into  two  canals,  the  scala  tympani 
and  the  scala  vestibuli,  corresponding  in  name  to  the  tym- 
panic and  vestibular  openings  of  the  cochlea.  The  semicircu- 
lar canals,  three  in  number — superior,  external  and  posterior 
— describe  arches  from  the  posterior  aspect  of  the  vestibule, 
communicating  by  both  their  extremities  with  that  cavity. 

The  membranous  labyrinth  consists  of  a  special  mem- 
brane lying  inside  the  bony  labyrinth  and  corresponding  in 
general  outline  to  the  walls  of  the  cavity.  It  is,  however,  sep- 
arated from  the  walls  by  perilymph,  and  encloses  a  similar 
fluid,  the  endolymph.  It  covers  the  sides  of  the  lamina  spir- 
alis in  the  cochlea  and  completes  the  septum,  besides  follow- 
ing the  wall  proper ;  and  on  one  side  it  sends  a  distinct  pro- 
cess from  the  tip  of  the  lamina  spiralis  to  the  wall  of  the  ca- 
nal, so  that  there  are  in  reality  three  divisions  of  the  lumen  of 
the  cochlea.  This  process  is  the  membrane  of  Reissner,  and 
the  third  canal  is  the  scala  media  the  true  membranous  coch- 
lea. (See  Fig.  91.) 

Termination  of  Auditory  Nerve. — The  membranous  laby- 
rinth, containing  and  being  suspended  in  fluid,  receives  the 
terminal  filaments  of  the  eighth  nerve  as  well  as  all  the  so- 
norous vibrations  intended  for  that  nerve.  When  the  audi- 
tory nerve  has  reached  the  base  of  the  internal  auditory 
meatus  it  enters  the  internal  ear  by  two  divisions,  one  for  the 
vestibule  and  semicircular  canals  and  the  other  for  the  coch- 
lea. The  vestibular  portion  again  subdivides,  sending  one 
branch  to  the  utricle  and  superior  and  horizontal  semicircular 


FUNCTIONS   OF  THE  COCHLEA  323 

canals,  and  another  to  the  saccule  and  posterior  semicircular 
canal.  The  fibers  of  the  eighth  nerve  spread  out  over  the 
inner  surface  of  the  membrane  to  end  in  a  way  somewhat 
obscure.  The  membrane  is  lined  internally  by  epithelium 
whose  character  differs  in  different  areas.  In  the  region  of 
distribution  of  the  vestibular  portion  of  the  nerve  the  cells 
are  of  two  kinds,  hair  cells  and  rod  cells.  From  the  inner 
ends  of  the  hair  cells  ciliated  processes  project  into  the  en- 
dolymph;  to  their  outer  ends  pass  the  axis  cylinders  of  the 
nerve  fibers,  though  the  exact  mode  of  connection  is  not 
clear.  The  rod  cells  are  much  more  numerous  than  the  hair 
cells,  but  their  precise  connection  with  audition  is  not  ap- 
parent. 

Upon  the  basilar  membrane  are  the  rods  of  Corti.  They 
consist  of  two  sets  of  pillars  of  varying  length,  slanting  to- 
ward each  other,  thus  leaving  at  their  base  a  space  which  be- 
comes a  canal  by  a  longitudinal  succession  of  these  pillars. 
There  are  supposed  to  be  about  4,500  elements  in  the  outer 
and  6,500  in  the  inner  set  of  these  rods.  Intimately  associ- 
ated with  the  pillars  are  large  numbers  of  hair  cells  with 
which  the  auditory  nerve  filaments  may  communicate;  it  is 
certain  that  these  filaments  are  closely  connected  in  some  way 
with  the  pillars. 

Functions  of  the  Semicircular  Canals. — The  use  of  these 
is  obscure.  Their  destruction  is  not  followed  by  interference 
with  hearing,  although  auditory  filaments  are  distributed 
to  some  parts  of  them.  Curiously  enough,  however,  this 
lesion  is  one  of  the  three  chief  ones  interfering  so  markedly 
with  equilibrium — the  phenomena  following  it  being  not  un- 
like those  sequent  upon  lesions  of  the  cerebellum  and  the  pos- 
terior white  columns  of  the  cord. 

Functions  of  the  Cochlea. — While  the  exact  mechanism  of 
the  production  of  auditory  impressions  is  unknown,  there 
seems  to  be  no  doubt  that  such  mechanism  takes  place  almost 
entirely  in  the  cochlea,  and  that  fibers  which  convey  to  the 
auditory  centers  impressions  of  sound  are  distributed  to  the 


324  THE  SENSES 

• 

organ  of  Corti  therein.  That  is  to  say,  loss  of  the  sense  of 
hearing  supervenes  upon  destruction  of  this  part  of  the  in- 
ternal ear.  In  physics  it  is  known  that  for  a  sound,  for  ex- 
ample of  a  piano  string,  to  be  heard  the  membrana  tympani 
must  vibrate  in  unison  with  the  sonorous  vibrations  of  the 
cord;  that  is,  "consonating  bodies"  repeat  sonorous  vibra- 
tions, giving  them  their  proper  pitch  and  quality.  It  has 
been  supposed  that  the  thousands  of  rods  of  Corti,  of  vary- 
ing length  and  size,  in  the  cochlea  are  made  to  vibrate  separ- 
ately or  in  correctly  associated  collections  (like  the  strings  of 
a  harp),  and  thus  reproduce  communicated  vibrations,  and 
so  -give  rise  to  impressions  which,  conveyed  by  the  auditory 
nerve  to  the  center,  are  there  recognized  as  sounds  of  differ- 
ent degrees  of  intensity,  pitch  and  quality.  This  theory  may 
be  true,  but  its  correctness  is  probably  beyond  the  range  of 
experimental  proof. 

While  the  usual  mode  of  conduction  of  sound  waves  to  the 
cochlea  is  through  the  external  ear,  they  may  reach  it  in 
other  ways,  as  through  the  bones  of  the  head,  or  through  the 
Eustachian  tube.  Nor  is  the  integrity  of  the  membrana  tym- 
pani actually  necessary  to  the  production  of  sound ;  although 
practically  speaking  a  person  in  whom  this  organ  is  de- 
stroyed is  deaf,  he  can  hear  if  the  ossicles  can  in  some  way 
be  placed  in  vibration  by  sound  waves,  as  by  the  intervention 
of  an  artificial  membrane.  Indeed  it  has  already  been  seen 
that  none  of  the  parts  of  the  external  or  middle  ear  are  actu- 
ally necessary  to  hearing.  They  are  only  accessory  conveni- 
ences for  the  better  transmission  of  impressions  to  the  fila- 
ments of  the  auditory  nerve. 

The  (so-called)  tensor  and  laxator  tympani  muscles 
make  tense  or  lax  the  membrana  tympani,  thus  influencing 
the  rapidity  and  amplitude  of  its  vibrations,  and  therefore 
the  pitch  and  intensity  of  the  sound.  The  stapedius  pre- 
vents too  great  movements  of  the  stapes.  The  free  com- 
munication of  the  air  in  the  tympanum  with  that  in  the  mas- 
toid  cells  and  pharynx  insures  an  approximately  constant 


THE  PRODUCTION  OF  THE  VOICE  325 

internal  pressure  upon  the  membrane,  and  thus  precludes  ac- 
cidents which  would  otherwise  interfere  with  its  proper  vi- 
bration. 

The  auditory  center  in  man  is  in  the  first  and  second  tem- 
poral convolution  of  the  temporo-sphenoidal  lobe. 

Briefly  then,  the  physiology  of  hearing  is  as  follows: 
Sound  waves  collected  by  the  pinna  enter  the  external  audi- 
tory canal  and  impinge  upon  the  membrana  tympani.  The 
drum  is  thus  set  to  vibrating  and  communicates  its  move- 
ments to  the  ossicles,  which  in  turn  hand  them  over  through 
the  fenestra  ovalis  to  the  fluids  of  the  internal  ear,  through 
which  media  they  reach  the  auditory  filaments,  are  conducted 
to  the  brain  and  given  proper  recognition. 

The  Production  of  the  Voice. 

The  production  of  the  voice  is  not  connected  with  the  spe- 
cial senses,  but  its  consideration  will  be  introduced  here  for 
the  sake  of  convenience. 

The  Larynx  is  the  organ  of  voice.  It  is  a  cavity  closed 
except  for  its  openings  above  and  below.  It  consists  of  four 
cartilages — cricoid,  thyroid  and  two  arytenoid — joined  to- 
gether by  ligaments  and  muscles.  The  vocal  cords  are  at- 
tached posteriorly  to  the  bases  of  the  movable  arytenoid  car- 
tilages and  anteriorly  to  the  angle  between  the  alae  of  the  thy- 
roid. The  muscles  serve  to  move  the  cartilages  and  thus  to 
separate  or  approximate  and  to  render  lax  or  tense  the  vocal 
cords. 

Production  of  Sound. — The  human  voice  is  produced  by 
vibrations  of  the  vocal  cords,  which  vibrations  are  set  up  by 
currents  of  expired  air. 

Movements  of  the  Vocal  Cords. — These  are  those  taking 
place  (i)  in  respiration,  and  (2)  during  vocalization. 

i.  In  Respiration. — When  the  cords  are  "passive"  they  are 
approximated  anteriorly  and  separated  posteriorly,  so  that 
the  interval  between  them  (rima  glottidis)  is  triangular.  This 


THE  SENSES 

interval  becomes  a  little  wider  during  inspiration  and  a  little 
narrower  during  expiration. 

2.  In  Vocalization. — The  production  of  sound  in  the  larynx 
involves  an  approximation  of  the  cords  and  an  increase  in 
their  tension.  They  are  made  more  nearly  parallel  by  the 
approach  of  the  arytenoids  to  each  other,  and  the  rima  glot- 
tidis  assumes  the  shape  of  a  mere  chink.  The  tenser  the 
cords,  the  higher  the  note  produced ;  usually  also  the  closer 
the  cords  are  brought  together,  the  higher  the  note.  The 
range  of  the  voice  depends  principally  on  the  degree  of  ten- 
sion which  the  cord  can  be  made  to  assume. 

Varieties  of  Vocal  Sounds. — These  are  mainly  (i)  mo- 
notonous, (2)  transitional,  (3)  musical. 

1.  In  monotonous  sounds  the  notes  have  all  nearly  the 
same  pitch,  as  in  reading. 

2.  In  transitional  sounds  there  is  a  gradual  change  in  the 
tension  and  approximation  of  the  cords,  so  that  the  notes  be- 
come successively  higher  or  lower,  as  in  the  howling  of  a 
dog. 

3.  In  musical  sounds  the  vocal  cords  have  a  definite  num- 
ber of  vibrations  for  each  successive  note — a  number  corres- 
ponding to  the  production  of  that  note  in  the  musical  scale. 

The  range  of  the  average  human  voice  is  from  one  to 
three  octaves.  The  highest  and  lowest  notes  of  females  are 
about  one  octave  higher  than  the  corresponding  notes  of 
males.  The  chief  difference  between  male  and  female 
voices  is,  therefore,  one  of  pitch;  but  they  also  differ  materi- 
ally in  tone.  The  difference  in  pitch  is  a  result  of  the  differ- 
ent length,  and  therefore  the  different  rate  of  vibration,  of 
the  cords  in  the  two  sexes.  The  female  cords  are  about 
two-thirds  the  length  of  the  male. 

Before  puberty  the  male  larynx  resembles  the  female,  but 
at  that  period  the  alae  of  the  thyroid  becomes  more  promi- 
nent in  the  male  and  the  cords  increase  in  length,  thus  ac- 
counting for  the  change  of  voice. 

In  old  age  control  of  the  musculature  of  the  larynx  is 


SPEECH  327 

partly  lost,  the  cords  become  altered  and  the  cartilages  ossify. 
These  circumstances  make  the  voice  weak  and  unsteady. 

Speech. — Modifications  and  alterations  of  the  sounds  pro- 
duced in  the  larynx  during  and  after  their  production  result, 
under  the  influence  of  the  sensorium,  in  articulate  speech. 
These  modifications  are  made  chiefly  by  the  tongue,  teeth, 
and  lips. 

The  speech  sounds  are  divided  into  vowels  and  consonants. 
The  distinction  is  that  the  vowel  sounds  are  generated  in  the 
larynx,  while  the  consonant  sounds  are  produced  by  altera- 
tions in  the  current  of  air  above  the  larynx,  and  cannot  be 
pronounced  except  constantly  with  a  vowel.  The  current  is 
modified  mainly  by  the  tongue  and  teeth  in  the  formation  of 
linguals  and  dentals,  by  the  cavity  of  the  nose  in  case  of  na- 
sals, and  by  changes  in  the  shape  and  size  of  the  oral  cavity 
in  the  production  of  other  sounds. 

Nervous  Supply  of  the  Larynx. — The  superior  laryngeal 
branch  of  the  teeth  is  the  sensory  nerve,  which  guards  the 
glottis  to  prevent  the  entrance  of  foreign  bodies.  Impres- 
sions made  on  the  filaments  of  this  nerve  are  reflected 
through  the  medulla  and  inferior  laryngeal  branch  of  the 
tenth  to  the  muscles  which  close  the  glottis.  The  inferior 
laryngeal  also  innervates  the  muscles  that  vary  the  tension 
of  the  cords,  and  the  superior  laryngeal  keeps  the  mind  in- 
formed of  the  state  of  the  muscles  and  of  the  necessity  for 
forced  expiration  or  coughing. 


•*»-••  I 

CHAPTER  XIII. 
REPRODUCTION. 

VERY  many  facts  in  our  knowledge  of  reproduction  de- 
pend on  observations  made  upon  lower  animals,  but  there  is 
sufficient  analogy  between  the  known  facts  connected  with 
human  reproduction  and  development  and  those  of  the  same 
stages  in  other  groups  of  beings  to  enable  us  to  present,  as  at 
least  approximately  accurate,  certain  broad  principles  regard- 
ing the  process  as  it  pertains  to  the  human  race. 

In  order  that  a  human  being  may  be  brought  into  exist- 
ence it  is  necessary  that  there  be  a  union  of  the  male  ele- 
ment, the  spermatozoon,  and  the  female  element,  the  ovum. 
Both  these  sexual  cells  are  developed  from  epithelium — the 
spermatozoon  from  that  of  the  seminiferous  tubules  of  the 
male,  and  ovum  from  the  germinal  layer  of  the  ovary. 

In  what  follows  reference  will  be  had  to  reproductive  pro- 
cesses in  the  human  being. 

Spermatozoa. — Human  spermatozoa  (Fig.  92)  are  elon- 
gated bodies,  about  one  five-hundredth  of  an  inch  in  length, 
and  consist  of  three  parts,  head,  mid-portion  and  tail.  The 
last-named  part  is  about  four-fifths  the  length  of  the  entire 
spermatozoon.  The  head  is  egg-shaped  and  much  the  thick- 
est part  of  the  element.  A  slender  filament,  the  axial  fiber, 
extends  throughout  its  length  from  head  to  tail  and  projects 
slightly  beyond  the  latter.  Spermatozoa  are  possessed  of 
wonderful  vitality.  They  live  for  several  weeks  in  the  geni- 
tal passages  of  the  female.  In  the  male  genital  passages  they 
may  live  for  months  in  a  quiescent  state.  The  nucleus  is  the 
fertilizing  agent.  Spermatozoa  are  also  remarkable  for 
their  power  of  locomotion,  which  is  effected  by  lashing? 
and  rotary  movements  of  the  tail. 

328 


OVA 


329 


Ova. — The  ovum  (Fig.  93),  or  female  sexual  cell,  is  the 
largest  cell  to  be  found  in  the  human  body.  Its  diameter  is 
about  M.25  of  an  inch.  Its  structure  is  that  of  a  typical  cell. 
When  the  ovary  is  developing  a  part  of  its  covering  epithel- 


k 

m 


2 


Ttl 


\ 


FIG.  92. — Spermatozoa. 


i,  human  (  X  600),  the  head  seen  from  the  side;  2,  on  edge;  k,  head;  m. 
middle  piece;  f,  tail;  e,  terminal  filament;  3,  from  the  mouse;  4,  bothriocephalus 
latus;  5,  deer;  6,  mole;  7,  green  woodpecker;  8,  black  swan;  9,  from  a  cross  be- 
tween a  goldfinch  (m.)  and  a  canary  (f.) ;  10,  from  cobitis.  (Landois.") 

ium  dips  down  into  the  substance  of  the  organ  and  become? 
walled  off  by  the  union  of  the  surface  cells  above  it.  A 
part  of  this  ball  of  epithelium  becomes  the  ovum,  and  a  part 
the  Graafian  follicle  for  that  ovum.  The  youngest  ova  are 
thus  found  nearest  the  surface  of  the  ovary.  The  cell  has  an 
enveloping  membrane,  the  vitelline  membrane,  a  protoplasm, 
the  mtellus,  a  nucleus,  the  germinal  vesicle,  and  a  nucleolus, 


330 


REPRODUCTION 


the  germinal  spot.  Outside  the  ovum,  but  not  strictly  a  part 
of  it,  is  the  zona  pellucida,  a  transparent  envelope,  and  out- 
side the  zona  pellucida  a  collection  of  cells,  the  corona  radi- 
ata.  The  perivitelline  space  is  between  the  ovum  proper  and 
the  zona  pellucida.  The  zona  presents  a  radial  striae,  which 
may  facilitate  the  entrance  of  the  spermatozoon. 

Ova  are  capable  of  being  impregnated  as  long  as '7-9  days 
after  their  discharge  from  the  ovary.     Their  formation  be- 
gins early  in  fetal  life.    The  ovum  possesses  no  power  of  in- 
dependent motion.     It  is  pass- 
ive in  fecundation ;  it  is  sought 
by  the  male  element.    Its  vitel- 
lus,     or    yolk     (protoplasm), 
contains    nutritive    non-living 
material,    deutoplasm,    whose 
function    is    to    furnish    food 
substance  to  the  impregnated 
ovum  until  the  fetal  circulation 
is  established.     Deutoplasm  in 
the  human  ovum  is  scarcely  to 

,-,  ,_,  be  distinguished  from  the  liv- 

FIG.  93.— Ovum.    (From  Yeo     .  &.  11- 

after  Robin.)  mg  protoplasm,  though  in  the 

ova  of  birds,  e.  g..  it  is  clearly 

a,  zona  pellucida  and  yitelhne  mem-  i  •  • 

brane;  b,  yolk;  c,  germinal  vesicle  or  marked  Off,  and  Constitutes  the 
nucleus;   a,  germinal  spot  or  nucleo-  •       r     n         r    .1 

lus;  e,  interval  left  by  the  retraction  main    bulk   of    the   mature    egg, 

of  the  viteHus  from  the  zona  pellucida.  c;_     ,^     developing     embryo 


receives  no  blood  from  the  mother. 

Graafian  Follicles.-The  Graafian  follicles  are  directly 
concerned  in  the  development  and  maturation  of  ova.  These 
are  small  vesicles  in  the  cortical  ovarian  substance  sur- 
rounded by  a  capsule  of  thickened  ovarian  stroma,  the 
tunica  vasculosa.  Inside  the  tunica  vasculosa,  lining  the 
spherical  cavity  of  the  vesicle,  are  several  layers  of  epithelial 
cells  making  up  the  membrana  granulosa.  The  cavity  is 
filled  with  an  albuminous  liquid,  the  liquor  folliculi.  At  one 
point  in  its  circumference  the  membrana  granulosa  is  much 


GRAAFIAN  FOLLICLES 


331 


thickened,  and  in  this  thickened  portion  is  imbedded  the 
ovum,  The  epithelial  cells  of  the  membrana  completely  sur- 
round the  ovum,  constituting  the  discus  proligerus.  The 


FIG.  94. — Section  of  the  ovary  of  a  cat,  showing  the  origin  and  devel- 
opment of  Graafian  follicles.     (From  Yeo  after  C'adiat.) 

a,  germ  epithelium;  b,  Graafian  follicle  partly  developed;  c,  earliest  form  of 
Graafian  follicle;  d,  well-developed  Graafian  follicle;  e,  ovum;  f,  vitelline  mem- 
brane; g,  veins;  h,  i.,  small  vessels  cut  across. 

cells  of  the  discus  next  the  ovum  have  their  long  axes  at 
right  angles  to  the  circumference  of  the  egg,  and  this  layer 


332  REPRODUCTION 

is  the  corona  radiata  already  mentioned.  The  zona  pel- 
lucida  is  just  underneath  the  corona. 

Usually  a  Graaiian  follicle  contains  only  one  ovum.  The 
follicles  and  their  contained  ova  begin  to  be  formed  early  in 
fetal  life.  Probably  none  are  newly  formed  after  the  child 
is  two  years  old,  but  they  are  undeveloped  before  puberty. 
It  is  estimated  that  some  72,000  follicles  and  ova  exist  in 
the  two  ovaries  of  the  average  woman;  but  of  these  not 
more  than  400  reach  full  development,  the  others  undergoing 
retrograde  changes  and  disappearing. 

Up  to  puberty  the  follicles  and  ova  are  small,  but  at  that 
time  some  of  them  begin  to  enlarge,  and  at  more  or  less 
regular  intervals  one  of  these  follicles  bursts  and  allows  the 
escape  of  its  contained  ovum  into  the  fimbriated  extremity  of 
the  Fallopian  tube — a  process  known  at  ovulation.  Previ- 
ous to  its  rupture  the  Graafian  follicle  has  been  enlarging. 
It  is  always  located  in  the  cortical  part  of  the  ovary,  but  it 
may  now  not  only  form  a  distinct  protrusion  above  the  sur- 
face of  the  organ,  but  may  by  its  size  encroach  upon  the  me- 
dullary portion.  It  may  at  this  time  have  a  diameter  of  half 
an  inch.  Meantime  the  more  superficial  part  of  the  tunica 
vasculosa  has  been  undergoing  fatty  degeneration,  has  lost 
its  blood  supply  and  become  very  thin.  Hlere  rupture  oc- 
curs, and  the  mature  ovum,  ready  for  impregnation,  escapes 
upon  the  surface  of  the  ovary. 

Corpus  Luteum. — When  the  ovum  has  been  extruded  hem- 
orrhage occurs,  filling  the  empty  follicle  with  blood.  By 
contraction  of  the  extra-vesicular  adjacent  tissue  the  walls 
of  the  Graafian  follicle  become  folded  into  the  cavity.  Soon 
proliferation  of  the  cells  of  the  follicular  wall  takes  place 
into  the  blood  clot,  vascular  loops  are  formed,  and  the  tunica 
vasculosa  itself  becomes  greatly  hypertrophied.  The  clot 
.later  disappears  and  the  mass  then  has  a  yellowish  color  and 
is  known  as  the  corpus  luteum. 

Whether  or  not  the  ovum  that  escaped  from  the  follicle 
which  was  the  antecedent  of  any  given  corpus  luteum  was 


CORPUS  LUTEUM 


333 


impregnated,  has  an  influence  upon  the  growth  of  that  cor- 
pus. If  the  ovum  failed  of  fecundation  the  corpus  luteum 
will  reach  its  highest  development  in  about  fifteen  days,  and 
will  then  assume  the  character  of  cicatrical  tissue  and  be  ab- 
sorbed in  a  few  weeks.  If  the  ovum  is  fecundated,  the  cor- 
pus luteum  will  increase  in  size  for  some  three  months,  until 
it  may  be  half  the  size  of  the  ovary.  At  labor  it  has  been  re- 
duced to  a  white  cicatrix,  which  probably  persists  as  a  small 
nodule  throughout  life.  The  differences  between  the  cor- 
pora lutea  pf  menstruation  and  pregnancy  are  shown  by  the 
following  table  from  Dalton: 


Corpus  Luteum 
of     Menstruation. 


Corpus  Luteum 
of     Pregnancy. 


At  the  end  of 
three  weeks. 

One  month. 


Two  months. 


Four  months. 


Six  months. 


Nine  months. 


Three-quarters  of  an  inch  in  diameter;  central 
clot  reddish ;  convoluted  wall  pale. 


Smaller ;  convoluted 
wall  bright  yellow ;  clot 
still  reddish. 

Reduced  to  the  con- 
dition of  an  insignificant 
cicatrix. 


Absent    or    unnotice- 
able. 


Absent. 


Absent. 


Larger;  convoluted  wall 
bright  yellow;  clot  still 
reddish. 

Seven-eighths  of  an  inch 
in  diameter ;  convoluted ; 
wall  bright  yellow;  clot 
perfectly  decolorized. 

Seven-eighths  of  an  inch 
in  diameter;  clot  pale  and 
fibrinous;  convoluted  wall 
dull  yellow. 

Still  as  large  as  at  the 
end  of  second  month;  clot 
fibrinous ;  convoluted  wall 
paler. 

Half  an  inch  in  diame- 
ter; central  clot  converted 
into  a  radiating  cicatrix; 
external  wall  tolerably 
thick  and  convoluted,  but 
without  any  bright  yellow 
color. 


334  REPRODUCTION 

Maturation. — But  previous  to  its  discharge  from  the 
Graafian  follicle,  the  ovum  undergoes  certain  changes — a 
ripening  process — whereby  it  is  made  ready  to  receive  and 
be  impregnated  by  the  spermatozoon.  This  maturation  con- 
sists in  the  discharge  from  the  cell  proper  of  a  part  of  its 
nucleus  and  a  part  of  its  protoplasm.  The  nucleus 
moves  toward  the  periphery,  and  the  perinuclear 
membrane  is  lost.  As  the  nucleus  approaches  the  surface 
of  the  egg  it  undergoes  karyokinesis,  and  a  part  of  it,  to- 
gether with  a  little  surrounding  protoplasm,  is  extruded  and 


FIG.  95. — The  'fertilized  ovum,  or  blastophere.     (Kirkcs.) 

finds  itself  in  the  perivitelline  space.  This  is  the  first  polar 
body.  A  second  polar  body  is  likewise  later  discharged  by 
karyokinetic  division.  (See  Fig.  95.) 

The  object  of  this  extrusion  and  the  final  fate  of  the  polar 
bodies  are  matters  of  speculation.  That  portion  of  the  nu- 
cleus which  remains  after  the  polar  bodies  have  been  thrown 
off  finds  its  way  back  to  the  center  of  the  ovum.  It  soon  de- 
velops a  covering  membrane,  and  is  now  the  female  pronu- 
cleus,  ready  for  union  with  the  male  pronucleus.  It  is  about 
the  time  of  the  completion  of  this  process  that  the  follicle 
ruptures  and  the  discharge  of  the  ovum — ovulation — occurs. 

Ovulation. — It  is  supposed  that  from  puberty  to  the  meno- 
pause one  (or  more?)  ovum  is  discharged  at  tolerably  regu- 


MENSTRUATION  335 

lar  intervals  of  about  four  weeks.  It  should,  and  usually 
does,  enter  the  outer  end  of  the  Fallopian  tube,  to  be  con- 
veyed toward  the  uterus.  Obviously  only  a  few,  and  some- 
times none,  are  ever  impregnated.  Should  the  ovum  fail  to 
reach  the  uterus  and  become  fecundated,  ectopic  gestation 
will  be  the  result. 

The  patent  fimbriated  extremity  of  the  tube  may  grasp 
the  ovary  at  the  time  of  rupture  of  the  Graafian  follicle,  but 
this  is  not  probable.  One  of  the  tubal  fimbriae  is  attached  to 
the  outer  extremity  of  the  ovary  and  has  on  its  surface  a 
small  linear  depression  lined  by  ciliated  epithelium  and  lead- 
ing to  the  tube.  The  ovum  very  likely  in  most  cases  drops 
into  this  depression,  and  the  -influence  of  the  cilia  is  to  carry 
it  toward  the  tube. 

Menstruation. — Usually  between  the  fourteenth  and  sev- 
enteenth years  of  female  life  menstruation  begins.  It  is  a 
discharge  of  blood,  epithelium  and  other  parts  of  the  mu- 
cous membrane  of  the  uterine  cavity,  together  with  mucus 
from  the  glands  of  the  uterus  and  vagina.  About  the  be- 
ginning of  menstrual  life  there  are  marked  changes  in  bodily 
development,  Graafian  follicles  enlarge  and  begin  to  approach 
the  surface,  ovulation  is  begun,  and  the  female  is  capable  of 
being  impregnated. 

In  most  cases  menstruation  occurs  at  regular  intervals  of 
twenty-eight  days.  The  function  is  suspended  during  preg- 
nancy and  usually  during  lactation.  When  it  is  first  estab- 
lished it  is  frequently  irregular  in  its  occurrence  for  several 
months ;  a  like  irregularity  usually  accompanies  the  cessation 
of  the  function  between  the  fortieth  and  fiftieth  years — when 
the  menopause,  or  climacteric,  is  established.  The  normal 
female  may  be  impregnated  during  menstrual  life,  but  not 
before  or  after. 

The  average  length  of  time  for  which  the  menstrual  flow 
continues  is  four  days.  There  are  many  exceptions  in  both 
directions  for  different  women,  but  the  time  for  any  one 
woman  probably  varies  little  under  normal  conditions.  The 


REPRODUCTION 

discharge  for  each  period  averages  some  five  ounces.  It  does 
not  usually  coagulate,  on  account  of  the  presence  of  alkaline 
mucus.  For  five  or  six  days  preceding  the  flow,  the  uter- 
ine mucous  membrane  gradually  thickens,  the  glands  be- 
come longer  and  more  tortuous,  the  connective  tissue  cells 
multiply  and  the  blood-vessels  are  greatly  increased  in-  size. 
This  is  apparently  a  preparation  for  the  reception  of  the  im- 
pregnated ovum.  A  short  time  before  the  flow  begins  there 
is  hemorrhage  into  the  subepithelial  tissue,  possibly  by  dia- 
pedesis,  possibly  by  rupture.  In  a  day  or  so  the  super jacent 
mucous  membrane  becomes  disintegrated  and  is  discharged 
with  the  included  parts  of  the  glands.  The  underlying  ves- 
sels, being  thus  exposed,  rupture  and  the  sanguineous  dis- 
charge carries  away  the  debris. 

For  three  or  four  days  subsequent  to  the  cessation  of  the 
flow  the  uterine  mucosa  is  being  repaired.  The  deeper  lay- 
ers, including  the  deeper  portions  of  the  glands,  were  not 
cast  off,  and  the  whole  is  reconstructed  from  the  intact  parts. 
Following  the  reconstructive  period  there  is  a  stage  of  qui- 
escence lasting  some  two  weeks,  until  six  or  seven  days  prior 
to  the  next  menstruation. 

At  the  beginning  of  each  menstrual  flow  there  is  general 
congestion  of  the  pelvic  viscera  and  mammary  glands,  ac- 
companied usually  by  headache  and  a  sense  of  pelvic  oppres- 
sion. The  congestion  and  discomfort  begin  to  disappear 
when  the  flow  is  established. 

Ovulation  probably  in  most  cases  takes  place  just  before 
the  menstrual  flow  begins,  but  neither  occurrence  is  depen- 
dent upon  the  other.  Ovulation  has  frequently  been  shown 
to  take  place  in  the  inter-menstrual  period,  but  the  conges- 
tion of  the  reproductive  organs  incident  to  menstruation 
probably  hastens  the  rupture  of  any  -Graafian  follicle  which 
at  that  time  happens  to  be  near  the  completion  of  its  devel- 
opment. 

The  relations  between  ovulation,  menstruation  and  im- 
pregnation are  not  definitely  determined.  Pregnancy  lasts 


IMPREGNATION  337 

for  ten  lunar  months  and  dates  from  the  time  of  impreg- 
nation (conception),  but  that  time  cannot  in  any  case  be 
fixed  upon  with  precision.  The  vitality  of  the  ovum  is 
thought  not  to  last  longer  than  seven  days  unless  impreg- 
nated, and  if  impregnation  is  to  occur,  it  must  take  place 
within  the  first  week  after  ovulation.  Since,  therefore,  ovu- 
lation  and  menstruation  usually  occur  together,  and  since  im- 
pregnation probably  occurs  about  the  beginning  of  menstru- 
ation, we  reckon  from  the  first  day  of  the  last  menstruation 
280  days  forward  to  determine  the  probable  time  of  labor. 
This  is  equivalent  to  adding  nine  calendar  months  and  seven 
days  to  the  first  day  of  the  last  menstrual  period.  It  is  evi- 
dent that  this  calculation  at  best  gives  only  the  approximate 
time. 

While  fertilization  probably  occurs  at  the  time  mentioned, 
the  spermatozoon  effecting  fecundation  may  have  been  in  the 
female  genital  tract  for  weeks.  Its  vitality  here  is  so  pro- 
longed that  the  time  of  its  deposit  with  reference  to  men- 
struation very  probably  has  little  to  do  with  whether  or  not 
conception  shall  occur. 

Impregnation. — The  term  impregnation,  or  fertilisation, 
or  fecundation,  is  used  to  signify  that  union  of  the  male  and 
female  sexual  cells  which  makes  possible  the  development 
of  a  new  human  being.  Normally  impregnation  takes  place 
in  the  Fallopian  tube,  and  almost  always  in  the  outer  third. 
The  male  element,  the  spermatozoon,  seeks  and  penetrates 
the  female  element,  the  ovum.  It  is  the  blending  of  the  nu- 
clei (pronuclei)  which  is  essential.  Spermatozoa  in  large 
numbers  swarm  around  the  ovum  and  several  at  least  enter 
the  perivitelline  space.  Only  one,  however,  is  destined  usu- 
ally to  enter  the  ovum.  As  it  approaches  the  vitelline  mem- 
brane, head  first,  the  protoplasm  of  the  ovum  swells  up  into 
a  prominence  to  meet  it.  The  fertilizing  spermatozoon  makes 
its  way  through  the  vitelline  membrane,  losing  its  tail  in  the 
passage,  and  becomes  the  male  pronucleus.  The  female 
pronucleus  now  advances  from  its  central  position  to  meet 

22 


REPRODUCTION 
\ 

the  male  element,  and  they  coalesce  to  become  the  segmenta- 
tion nucleus.  Impregnation  has  now  taken  place.  The  seg- 
mentation nucleus  represents  a  new  being.  It  contains  ana- 
tomical elements  from  both  parents,  and  it  is  not  surprising 
that  the  child  should  resemble  both,  anatomically  and  other- 
wise. 

The  term  "ovum"  has  so  far  been  used  to  signify  the  un- 
impregnated  sexual  cell  discharged  from  the  female  ovary. 
It  is  also  used  to  signify  the  fertilized  cell,  and  is  in  fact 
often  applied  without  much  precision  to  the  product  of  con- 
ception at  almost  any  stage  of  its  intrauterine  development. 

The  fertilized  ovum  is  carried  through  the  tube  to  the 
uterus,  arriving  there  some  seven  days  after  its  fecundation. 
In  its  passage  it  becomes  covered  with  a  coating  of  albu- 
minous material.  This  layer  is  probably  impervious  to  sper- 
matozoa— which  fact  may  account  for  the  practical  univer- 
sality of  fecundation  in  the  outer  part  of  the  tube,  if  at  all. 
The  coating  corresponds  to  the  white  of  an  egg,  in  that  it 
penetrates  the  perivitelline  membrane  and  furnishes  nutritive 
material  to  the  vitellus.  On  reaching  the  uterus  the  ovum 
becomes  attached  to  and  covered  by  the  thickened  mucous 
membrane  of  that  organ  in  a  way  to  be  noted  presently. 
Here  it  remains  until  expelled  during  parturition. 

Segmentation. — As  soon  as  union  of  male  and  female 
pronuclei  has  taken  place,  cleavage  of  the  ovum  begins.  The, 
nucleus  (segmentation  nucleus)  and  protoplasm  divide  kary- 
okineses  to  form  two  nearly  similar  cells.  These  two  divide 
into  four,  these  four  into  eight  and  so  on,  till  a  large  number 
of  cells  occupy  the  vitelline  space  and  are  all  surrounded  by 
the  perivitelline  membrane.  As  division  proceeds,  cells  ar- 
range themselves  around  others,  so  that  the  former  occupy 
the  circumference  and  the  latter  the  center  of  the  vitelline 
cavity.  Later,  while  the  outer  cells  constitute  a  layer  cover- 
ing the  entire  inner  surface  of  the  perivitelline  membrane,  the 
inner  cells  group  to  form  a  mass  which  is  in  contact  with  the 
outer  layer  at  one  point  only — like  a  ball  lying  in  a  relatively 


SEGMENTATION 


339 


large  hollow  sphere.  The  space  thus  left  between  the  two 
kinds  of  cells  is  called  the  segmentation  cavity.  Soon  the  sur- 
rounding cells  become  attenuated  (Rauber's 'cells)  and  dis- 


FIG.  96.— Sections  of  the  ovum  of  a  rabbit,  showing  the  formation  o«t 
the  blastodermic  vesicle.    (From  Yeo  after  E.  Van  Beneden.) 

a,  b,  c,  d,-  are  ova  in  successive  stages  of  development;  z.p.,  zona  pellucida; 
ect,  ectomeres,  or  outer  cells;  ent,  entomeres,  or  inner  cells. 

appear.  Their  place,  as  a  surrounding  envelope,  is  taken 
by  some  of  the  cells  of  the  inner  layer.  This  second  sur- 
rounding layer  is  the  epiblast,  or  ectoderm;  the  surrounded 
mass  is  the  hypoblast,  or  entoderm. 


340 


REPRODUCTION 


Before  long  the  entoderm  spreads  out  over  a  larger  area, 
and  from  it  and  from  the  ectoderm  is  developed  a  layer  of 
cells,  the  mesoblast,  or  mesoderm,  which  occupies  a  position 
between  the  other  two  layers.  The  three-layered  germ  is 
now  the  blastodermic  vesicle,  or  the  gastrula,  and  its  cavity 
is  the  archenteron,  or  celenteron.  From  these  three  germ 
layers  are  developed  all  the  parts  of  the  body  by  the  forma- 
tion of  folds,  ridges,  constrictions,  etc.,  and  by  various  meta- 
morphoses which  have  as  their  end  the  adaptation  of  struc- 
ture to  function. 

Derivatives  of  the  Germ  Layers. — According  to  Heisler 
these  are : 

From  the  ectoderm:  (i)  The  epidermis  and  its  append- 
ages, including  the  nails,  the  hair,  the  epithelium  of  the  se- 
baceous and  sweat  glands  and  the  epithelium  of  the  mam- 
mary gland.  (2)  The  infoldings 
of  the  epidermis,  including  the 
epithelium  of  the  mouth  and 
salivary  glands,  of  the  nasal 
tract  and  its  communicating  cav- 
ities, of  the  external  auditory 
canal,  of  the  anus  and  anterior 
urethra,  of  the  conjunctiva  and 
anterior  part  of  the  cornea,  the 
anterior  lobe  of  the  pituitary 
body,  the  crystalline  lens  and  the 
enamel  of  the  teeth.  (3)  The 
spinal  cord  and  brain  with  its 
outgrowths,  including  the  optic 
nerve,  the  retina  and  the  pos- 
terior lobe  of  the  pituitary  body. 
(4)  The  epithelium  of  the  inter- 
nal ear. 

From  the  entoderm:  The  epithelium  of  the  respiratory 
tract,  of  the  digestive  tract  (from  the  back  part  of  the  phar- 
ynx to  the  anus,  including  its  associated  glands,  the  liver  and 


FIG.  97.— Impregnated  egg. 

With  commencement  of  forma- 
tion of  embryo;  showing  the  area 
germinativa  or  embryonic  spot, 
the  area  pellucida,  and  the  primi- 
tive groove  and  streak.  (Kirkes 
after  Dalton.) 


DEVELOPMENT  OF   MESODERM  341 

pancreas),  of  the  middle  ear  and  Eustachian  tube,  of  the 
thymus  and  thyroid  bodies,  of  the  bladder  and  first  part  of 
the  male  urethra  and  of  the  entire  female  urethra. 

From  the  mesoderm:  (i)  Connective  tissue  in  all  its 
forms,  such  as  bone,  dentine,  cartilage,  lymph,  blood,  fibrous 
and  areolar  tissue;  (2)  muscular  tissue;  (3)  all  endothelial 
cells;  (4)  the  spleen,  kidney  and  ureter,  testicle  and  its  ex- 
cretory ducts,  uterus,  Fallopian  tube,  ovary  and  vagina. 

The  Embryonal  Area. — Soon  after  the  germ  reaches  the 
uterus  (probably)  there  appears  on  its  surface  an  oval  whit- 
ish spot,  the  embryonal  area.  The  impregnated  ovum  is  still 
in  the  shape  of  a  vesicle.  It  is  from  the  embryonal  area 
alone  that  the  body  is  developed.  The  other  parts  are  acces- 
sory. Longitudinal  division  of  this  area  is  supposed  to  give 
rise  to  twins  of  the  same  sex  and  of  almost  identical  struc- 
ture. Running  in  the  long  diameter  of  the  embryonal  area 
is  a  marking,  the  primitive  streak,  in  which  is  a  longitudinal 
depression,  the  primitive  groove.  (Fig.  97).-  These  sur- 
face markings  are  caused  by  thickening  of  the  ectoderm. 
(Fig.  98.) 

Development  of  Mesoderm. — It  is  about  this  time  that  the 
mesoderm  makes  its  appearance.  It  begins  under  the  primi- 
tive groove  and  extends  in  all  directions.  It  originates  from 
both  ectoderm  and  entoderm,  and  lies  between  them.  In  the 
median  line  the  three  layers  are  closely  united  to  each  other. 
(Fig.  98).  At  first  the  mesoderm  does  not  completely  em- 
brace the  germ,  but  is  deficient  opposite  the  embryonal  area. 

Fig.  94  shows  that  the  cells  of  the  mesoderm  make  up  a 
thickened  mass  near  the  median  line,  but  farther  away  they 
constitute  two  distinct  lamellae.  The  mass  near  the  median 
line  is  the  vertebral  or  axial  plate.  The  outer  of  the  lateral 
lamellae  is  the  somatic  mesoderm,;  the  inner  is  the  splanchnic 
mesoderm.  The  ectoderm  and  somatic  mesoderm  unite  to 
form  the  somatopleure ;  the  entoderm  and  splanchnic  meso- 
derm unite  to  form  the  splanchnopleure.  The  interval  left 
between  the  somatopleure  and  splanchnopleure  is  the 


342 


REPRODUCTION 


or  body  cavity.    (Fig.  98.)     The  great  serous  cavities  of  the 
body  are  developed  from  it. 

Beginning  Differentiation.— It  thus  appears  that  the  em- 


bryo is  beginning  to  develop  from  the  simple  vesicle  into 
specialized  parts. 

We  shall  notice  briefly  the  development  of  the  body  pro- 


NEURAL  CANAL 


343 


per,  and  the  extra-embryonic  accessory  structures,  the  um- 
bilical vesicle,  amnion,  allantois  and  placenta.  As  regards 
the  embryonic  body,  some  of  the  most  prominent  occurrences 


connected  with  its  development  consist  in  the  formation  of 
the  neural  canal,  chorda  dorsalis,  or  notochord,  and  meso- 
blastic  somites. 
Neural  Canal. — About  the  fourteenth  day,  along  under- 


344 


REPRODUCTION 


neath  the  primitive  groove,  the  cells  of  the  ectoderm  become 
thickened  to  form  the  medullary  plate.  The  edges  of  this 
longitudinal  plate  soon  begin  to  curl  up,  and  thus  form  the 
medullary  furrow,  or  groove.  (Fig.  99.)  The  margins  of 
the  adjacent  ectoderm  are  carried  up  with  the  curling  edges, 
and  constitute  the  medullary  folds.  Later  the  edges  of  the 
medullary  plate  meet  each  other,  and  join  to  form  a  closed 
canal,  the  neural,  or  medullary  canal.  The  edges  of  the 


a.O 


FIG.  100. — Transverse  section  through  dorsal  region  of  embryo  chick 

(45  hours). 

One-half  of  the  section  is  represented;  if  completed  it  would  extend  as  far  to 
the  left  as  to  the  right  of  the  line  of  the  medullary  canal  (Me).  A,  epiblast; 
C,  hypoblast,  consisting  of  a  single  layer  of  flattened  cells;  Me,  medullary  canal ; 
Pv,  protovertebra;  IV d,  Wolffian  duct;  So,  somatopleure;  Sp,  splanchnopleure; 
pp,  pleuroperitoneal  cavity;  eh,  riotochord;  ao,  dorsal  aorta,  containing  blood- 
cells;  v,  blood-vessels  of  the  yolk-sac.  (Kirkes  after  Foster  and  Balfour.) 

medullary  folds  unite  above,  so  that  the  neural  canal  comes 
to  lie  underneath  the  surface  ectoderm.  (Fig.  100.)  The 
neural  canal  is  the  forerunner  of  the  whole  nervous  system. 
Chorda  Dorsalis. — The  method  of  formation  of  the  chorda 
dorsalis,  or  notochord,  is  very  similar  to  that  of  the  neural 
canal.  It  is  a  solid,  instead  of  a  cylindrical,  longitudinal  col- 
lection of  cells,  extending  along  the  dorsal  aspect  of  the 
celom.  It  is  developed  from  the  entoderm.  A  thickening  of 
the  cells  of  this  layer  constitutes  the  chordal  plate.  Its  edges 
curl  up  in  a  direction  opposite  to  those  of  the  medullary  plate 


BODY  CAVITY  345 

and  carry  with  them  chordal  folds  of  the  entoderm.  When 
the  curling  edges  have  joined  to  form  a  solid  cylinder  of 
cells,  the  chordal  folds  unite  over  the  ventral  surface  of  the 
cylinder.  Figures  99  and  100  illustrate  these  facts.  The 
notochord  is  in  the  line  of  the  future  vertebral  bodies,  but  it 
is  not  developed  into  any  adult  structure. 

Somites. — These  are  masses  of  cells  developed  from  the 
axial  plates  of  the  mesoderm,  lying  parallel  with  and  on  each 
side  of  the  notochord.  (Fig.  100.)  They  are  in  segments, 
the  formation  of  which  begins  in  the  neck  and  proceeds 
caudad  and  cephalad.  They  are  sometimes  called  the  pro- 
tovertebra.  They  represent  the  primitive  vertebrae. 

The  body  begins  to  assume  shape  and  the  fetal  append- 
ages to  be  developed  at  the  same  time.  The  latter  are  for 
the  protection  and  nutrition  of  the  embryo.  The  essential 
parts  of  a  vertebrate  are  a  vertebral  column  with  a  neural 
canal  above  and  a  body  cavity  below  it.  The  body  cavity 
contains  the  alimentary  canal.  The  somites  representing  the 
vertebral  column  and  the  formation  of  the  neural  canal  have 
been  noticed. 

Body  Cavity. — At  first  the  embryo,  as  represented  by  the 
embryonal  area,  is  on  a  level  with  the  remaining  surface  of 
the  blastoderm.  Soon,  however,  there  appears,  marking  the 
head  of  the  embryo  and  with  its  concavity  backward,  a  cres- 
centic  folding  in  of  the  blastodermic  wall.  It  is  evident  on 
the  surface  as  a  simple  furrow.  This  tucking-in  finally  sur- 
rounds the  whole  embryonal  area,  and  the  surface  fissure, 
now  oval,  becomes  deeper  and  deeper,  until  those  portions 
of  the  wall  which  are  being  tucked  under  the  embryo  ap- 
proach each  other  on  its  ventral  aspect  and  divide  the  yolk 
into  two  communicating  cavities.  (See  Figs.  102  and  103.) 

The  layers  of  the  blastoderm  thus  folded  underneath  the 
embryo  are  the  visceral  plates.  They  form  the  boundaries 
of  a  cavity  which  still  communicates  in  front,  at  the  site  of 
the  future  umbilicus,  with  the  yolk-sac.  This  narrow  canal 
is  the  vitelline  duct,  and  the  two  cavities  communicating 


346  REPRODUCTION 

through  the  vitelline  duct  are  the  future  alimentary  canal 
and  the  yolk-sac,  or  umbilical  vesicle.  It  is  to  be  noticed 
that  the  visceral  plates  embrace  both  somatopleure  and 


FIG.  101. — Diagrammatic  section  showing  the  relation  in  a  mammal 

between  the  primitive  alimentary  canal  and 

the  membrane  of  the  ovum. 

The  stage  represented  in  this  diagram  corresponds  to  that  of  the  fifteenth  or 
seventeenth  day  in  the  human  embryo,  previous  to  the  expansion  of  the  allantois; 
c,  the  villqus  chorion;  a,  the  amnion;  a',  the  place  of  convergence  of  the  amnion 
and  reflexion  of  the  false  amnion;  a",  a",  outer  or  corneous  layer;  e,  the  head 
and  trunk  of  the  embryo,  comprising  the  primitive  vertebrae  and  cerebro-spinal 
axis ;  i,  i,  the  simple  alimentary  canal  in  its  upper  and  lower  portions.  Immedi- 
ately beneath  the  right  hand  *  is  seen  the  fetal  heart,  lying  in  the  anterior  part 
of  the  pleuroperitoneal  cavity;  v,  the  yolk-sac  or  umbilical  vesicle;  vi,  the  vitello- 
intestinal  opening;  u,  the  allantois  connected  by  a  pedicle  with  the  hinder  por- 
tion of  the  alimentary  canal.  (Kirkes  after  Quain.) 

splanchnopleure,  and  that  it  is  the  ectodermic  layers  of  the 
splanchnopleure  which  finally  join  to  form  the  gut  tract,  and 
the  somatopleure  which  forms  the  ventral  ancj  lateral  walls 


FETAL  MEMBRANES  347 

of  the  body  cavity.  The  gut  tract  has  the  shape  of  a  straight 
tube  occupying  the  long  axis  of  the  embryo  and  opening  into 
the  umbilical  vesicle. 

Fetal  Membranes. 

Umbilical  Vesicle. — The  umbilical  vesicle  represents  that 
part  of  the  vitellus  which  has  not  been  constricted  off  to 


FIGS.  102  AND  103. 

a,  chorion  with  villi.  The  villi  are  shown  to  be  best  developed  in  the  part  of 
the  chorion  to  which  the  allantois  is  extending;  this  portion  ultimately  becomes 
the  placenta;  b,  space  between  the  true  and  false  amnion;  c,  amniotic  cavity; 

d,  situation  of  the  intestine,  showing  its  connection  with  the  umbilical  vesicle; 

e,  umbilical  vesicle;  f,  situation  of  heart  and  vessels;  g,  allantois.    (Kirkes.) 

form  the  gut  tract.  (Figs.  101,  102,  103.)  It  furnishes 
nutriment  to  the  embryo  for  a  short  time  and  is  then 
largely  cut  off  from  the  body.  It  gradually  shrivels  (Figs. 
107,  108),  and  with  that  part  of  the  duct  external  to  the  ab- 
domen is  cast  off  either  before  or  at  parturition.  Vessels 
develop  in  its  walls  and  absorb  the  nourishment  in  it  to  be 
conveyed  to  the  embryo.  But  in  the  human  being  more  sat- 
isfactory arrangements  for  nutrition  are  soon  made  and  its 
function  ceases. 
Amnion. — When  the  embryo  has  become  depressed,  as  it 


348 


REPRODUCTION 


were,  into  the  substance  of  the  blastoderm,  and  while  the 
body  cavity  is  being  formed,  the  layers  of  the  somatopleure 
grow  up  over  the  embryo  to  meet  and  blend  dorsally.  (Figs. 
107,  108.)  The  two  layers  of  which  the  somatopleure  is 
composed  separate,  the  outer  forming  the  false  amnion  and 


FIG.  104. — Diagram  of 
fecundated  egg. 

a,  umbilical  vesicle;  b, 
amniotic  cavity;  c,  allan- 
tois.  (Kirkes  after  Dai- 
ton.) 


FIG.  105. — Fecundated  egg  with  allantois 
nearly  complete. 

a,  inner  layer  of  amniotic  fold;  b,  outer  layer  of 
ditto;  c,  point  where  the  amniotic  folds  come  in 
contact.  The  allantois  is  seen  penetrating  between 
the  outer  and  inner  layers  of  the  amniotic  folds. 
This  figure,  which  represents  only  the  amniotic 
folds  and  the  parts  within  them,  should  be  compared 
with  Figs.  99,  100,  in  which  will  be  found  the  struc- 
tures external  to  these  folds.  (Kirkes  after  Dalton.) 


the  inner  the  true  amnion.  The  false  amnion  now  coalesces 
with  the  original  vitelline  membrane  to  constitute  the  false 
chorion.  Evidently  there  is  thus  formed  a  closed  cavity,  the 
amniotic  cavity,  between  the  true  amnion  and  the  body  of 
the  embryo. 

At  first  the  amnion  and  the  embryo  are  in  close  contact, 
but  soon  the  cavity  begins  to  be  distended  with  the  fluid,  the 
liquor  amnii,  which  increases  until  it  reaches  a  considerable 
quantity.  It  affords  mechanical  protection  to  the  fetus  dur- 
ing intrauterine  life,  and  at  labor  serves  to  evenly  dilate  the 
cervix.  When  this  has  been  accomplished  is  the  usual  time 
at  which  the  sac  ruptures  and  the  liquor  amnii  escapes.  It 
also  supplies  the  fetal  tissues  with  water,  parts  of  it  being 
swallowed  from  time  to  time. 


THE  ALLANTOIS 


349 


The  cavity  between  the  false  amnion  and  the  true  amnion 
is  continuous,  with  the  body  cavity  at  the  umbilicus. 

Allantois. — The  allantois  grows  out  from  the  back  part  of 
the  intestinal  canal  into  the  celom  or  the  body  cavity.  (Figs. 


FIG.  106. — This  and  the  two  following  wood-cuts  are  diagrammatic 
views  of  sections,  through  the  developing  ovum,  showing  the  forma- 
tion of  the  membranes  of  the  chick.  (Yeo  after  Foster  and  Balfour.) 

A,  B,  C,  D,  E,  and  F,  are  vertical  sections  in  the  long  axis  of  the  embryo  at 
different  periods,  showing  the  stages  of  development  of  the  amnion  and  of  the 
yolk-sac;  /,  II,  III,  and  IV,  are  transverse  sections  at  about  the  same  stages  of 
development;  i,  ii,  and  in,  give  only  the  posterior  part  of  the  longitudinal  sec- 
tion to  show  three  stages  in  the  formation  of  the  allantois;  e,  embryo;  y,  yolk; 
pp,  pleuroperitoneal  fissure;  vt,  vitelline  membrane;  af,  amniotic  fold;  al,  allan- 
tois. 

104,  105.)  It  is  of  splanchnopleuric  origin.  It  soon  be- 
comes a  membranous  sac,  the  walls  of  which  are  very  vascu- 
lar. It  fills  the  space  between  the  two  amniotic  folds  and 
joins  the  false  amnion.  Its  vessels  thus  reach  the  chorion, 
which  is  already  establishing  vascular  connections  with  the 


350 


REPRODUCTION 


mother.     Finally  they  are  distributed  only  to  a  certain  part 
(placenta)  of  the  chorion ;  and  as  the  allantoic  vessels  anas- 


FIG.  107. 

e,  embryo;  a,  amnion;  a',  alimentary  canal;  vt,  vitelline  membrane;  af,  amniotic 
fold;  ac,  amniotic  cavity;  y,  yolk;  al,  allantois. 

tomose  more  and  more  freely  with  those  of  the  chorion,  the 
umbilical  vesicle  shrivels,  as  it  is  no  longer  needed.  The 
vessels  of  the  allantois  are  the  two  allantoic  arteries  and  the 


CHORION 


351 


same  number  of  allantoic  veins.    The  allantois  also  receives 
the  fetal  urine. 

As  the  true  placental  circulation  is  established  and  the  vis- 
ceral plates  close  the  abdominal  cavity,  the  allantois  is  con- 
stricted at  the  umbilicus  so  as  to  be  divided  into  two  parts. 


FIG.  108. — Diagrammatic  sections  of  embryo. 

Showing  the  destiny  of  the  yolk-sac,  ys.  vt,  vitelline  membrane;  pp,  pleuro- 
peritoneal  cavity;  ac,  cavity  of  the  amnion;  a,  amnion;  a',  alimentary  canal; 
ys,  yolk-sac. 

That  outside  the  body  shrivels  and  is  cut  away  with  the  um- 
bilical cord  at  birth,  while  that  inside  the  body  becomes  the 
first  part  of  the  male  and  the  whole  of  the  female  urethra, 
the  bladder  and  the  urachus. 

Chorion. — The  chorion  is  the  outer  surrounding  mem- 
brane of  the  embryo  after  the  appearance  of  the  amnion.  It 
consists  of  three  layers.  From  without  inward  these  are  the 


352  REPRODUCTION 

original  vitelline  membrane,  the  false  amnion  and  the  allan- 
tois.  The  allantois  has  been  seen  to  extend  around  between 
the  two  amniotic  folds  and  to  blend  with  the  outer. 
From  its  formation  from  these  several  membranes,  the  cho- 
rion  evidently  consists  of  the  outer  ectodermic,  inner  ento- 
dermic  and  intervening  mesodermic  strata. 

By  the  time  the  impregnated  ovum  reaches  the  uterus,  the 
chorion  (false  at  this  time)  has  numerous  spike-like  projec- 
tions— villi — over  its  whole  surface.  (Fig.  101.)  These  are 
at  first  non-vascular,  but  soon  become  vascular  by  the  pro- 
jection into  them  of  capillaries  from  the  vessels  of  the  allan- 
tois. These  capillaries  probably  absorb  nutrient  matter  se- 
creted by  the  uterine  glands.  .But  at  the  beginning  of  the 
third  month  the  villi  become  much  more  highly  developed 
over  a  certain  part  of  the  surface  of  the  chorion  than  at 
other  points,  and  a  more  intimate  relation  is  established  be- 
tween their  vessels  and  those  of  the  mother;  here  the  pla- 
centa is  to  be  formed. 

The  Decidua. — The  decidua  of  pregnancy  consists  of  the 
hypertrophied  mucous  membrane  lining  the  cavity  of  the 
uterus  and  reflected  at  a  certain  point  entirely  over  the  de- 
veloping ovum.  Before  the  ovum  reaches  the  uterus,  the 
mucous  membrane  of  the  latter  has  been  undergoing 
changes,  such  as  are  mentioned  under  Menstruation.  If  fe- 
cundation has  not  taken  place,  menstruation  occurs  and  the 
mucosa  is  discharged  under  the  name  of  the  decidua  men- 
strualis.  But  if  conception  has  occurred,  menstruation  does 
not  ensue  and  the  uterine  mucosa  becomes  much  more  thick 
and  spongy.  Whether  or  not  it  shall  be  discharged  as  the 
decidua  of  menstruation  or  be  retained  to  form  the  decidua 
of  pregnancy  is  probably  a  point  which  is  decided  while  the 
ovum  is  yet  in  the  tube. 

When  the  fecundated  ovum  reaches  the  uterus  it  becomes 
attached  to  the  mucous  membrane,  usually  a  little  to  one  side 
of  the  median  line  on  the  posterior  wall.  The  mucous  mem- 
brane extends  over  and  completely  envelops  it.  This  re- 


THE  DECIDUA 


353 


fleeted  portion  is  the  decidua  reflexa;  that  lining  the  whole 
uterine  cavity  is  decidua  vera,  while  that  part  of  the  decidua 
vera  intervening  between  the  ovum  and  the  uterine  wall  is 


FIG.  109. — Diagrammatic  view  of  a  vertical  transverse  section  of  the 
uterus  at  the  seventh  or  eighth  week  of  pregnancy. 


c,  c,  c' ,  cavity  of  uterus, 


the  decidua,  opening  at 


is,  which  becomes  the  cavity  of  the  ut^UUa, 

c,  c,  the  cornua,  into  the  Fallopian  tubes,  and  at  c'  into  the  cavity  of  the  cervix, 
which  is  closed  by  a  plug  of  mucus;  dv,  decidua  vera;  dr,  decidua  reflexa,  with 
the  sparser  villi  imbedded  in  its  substance;  ds,  decidua  serotina,  involving  the 
more  developed  chorionic  villi  of  the  commencing  placenta.  The  fetus  is  seen 
lying  in  the  amniotic  sac;  passing  up  from  the  umbilicus  is  seen  the  umbilical 
cord  and  its  vessels  passing  to  their  distribution  in  the  villi  of  the  chorion;  also 
the  pedicle  of  the  yolk-sac,  which  lies  in  the  cavity  between  the  amnion  and 
chonon.  (Kirkes  after  Allen  Thomson.) 

the  decidua  serotina  and  becomes  the  maternal  part  of  the 
placenta. 

23 


354  REPRODUCTION 

Of  course  there  is  at  first  a  considerable  cavity  left  be- 
tween the  reflex  and  the  vera,  but  as  the  embryo  increases  in 
size  the  space  becomes  smaller  and  is  obliterated  by  the  end 
of  the  fifth  month.  After  this  time  both  vera  and  reflexa 
undergo  retrograde  changes  due  to  pressure  and  become 
closely  attached  to  the  chorion.  They  are  discharged  with 
the  membranes  at  birth. 

Placenta. — The  placenta  is  the  organ  of  nutrition  for  the 
fetus  after  about  the  end  of  the  third  month.  Through  it  the 
vessels  of  the  fetus  and  those  of  the  mother  are  brought 
into  most  intimate  relations. 

It  has  been  said  that  the  villi  of  the  chorion  in  one  locality 
become  very  highly  developed.  This  is  at  the  site  of  the 
reflection  of  the  decidua  serotina  and  is  the  chorion  fron- 
dosum.  The  union  of  these,  with  certain  other  develop- 
ments, constitutes  the  placenta. 

The  decidua  serotina  becomes  very  spongy.  It  is  filled 
with  sinuses,  into  which  the  enlarged  villi  of  the  chorion 
frondosum  project.  The  sinuses  are  filled  with  maternal 
blood,  while  the  capillaries  of  the  villi  contain  fetal  blood. 
There  is  no  direct  connection  between  the  vessels  of  mother 
and  child,  but  the  thin  lining  of  the  villi  and  sinuses  allows 
free  interchange  of  materials  by  osmosis. 

It  seems  that  the  interchange  is  under  the  influence  of  two 
sets  of  cells,  each  disposed  in  a  single  layer — one  belonging 
to  the  maternal  and  the  other  to  the  fetal  part  of  the  pla- 
centa. These  layers  of  cells  are  situated  on  either  side  of 
the  membrane  of  the  villus.  They  seem  to  take  out  of  the 
maternal  blood  materials  needed  for  the  nutrition  of  the  fe- 
tus, and  out  of  the  fetal  blood  materials  which  require  re- 
moval. The  maternal  blood  performs  both  alimentary  and 
respiratory  functions  for  the  fetus. 

The  placenta  as  a  whole  is  discoid  in  shape.  Its  fetal  sur- 
face is  concave  and  covered  by  the  amnion.  The  mass  has  a 
diameter  of  4-5  in.,  and  a  thickness  of  half  an  inch.  The 
villi  receive  blood  from  the  allantoic  or  umbilical  arteries ;  it 
is  returned  by  the  umbilical  vein. 


UMBILICAL  CORD  355 

At  labor  uterine  contractions  detach  the  placenta  and  the 
decidua  and  expel  them  from  the  womb.  The  separation 
takes  place  in  the  deeper  part  of  the  maternal  placenta,  or 
decidua  serotina,  so  that  the  mass  discharged  represents  both 
the  fetal  and  maternal  portions.  The  vessels  entering  the 
sinuses  do  so  obliquely ;  consequently  uterine  contractions  at 
birth  very  effectually  check  the  hemorrhage  which  separa- 
tion of  the  placenta  occasions. 

Umbilical  Cord. — The  umbilical  cord  is  made  up  of  the 
vessels  which  convey  blood  between  the  placenta  and  fetus, 
together  with  the  remnants  of  the  umbilical  vesicle  and  allan- 
toic  stalk,  all  of  which  are  held  together  by  the  jelly  of 
Wharton,  a  species  of  connective  tissue. 

The  outgrowing  allantois  has  developed  in  it  the  two  al- 
lantoic  arteries  and  veins.  By  the  time  the  placenta  is 
formed  the  allantoic  stalk  has  become  much  elongated,  and 
the  allantoic  vessels  extend  into  the  fetal  placenta  (chorion 
f  rondosum)  and  become  now  the  umbilical  vessels.  The  two 
veins  blend  to  constitute  a  single  umbilical  vein,  but  the  ar- 
teries remain  separate.  The  vein  enters  the  fetal  body  at  the 
umbilicus,  passes  to  the  under  surface  of  the  liver  and  di- 
vides in  a  manner  to  be  noted  presently.  After  birth  the 
intra-abdominal  portion  atrophies,  and  is  the  round  liga- 
ment of  the  liver.  The  two  umbilical  arteries  issue  at  the 
umbilicus.  Their  intra-abdominal  portions  are  the  fetal  hy- 
pogastric  arteries. 

The  average  length  of  the  umbilical  cord  is  about  twenty- 
one  inches.  It  appears  to  be  twisted  on  account  of  the  spiral 
course  of  its  relatively  long  arteries.  It  is  usually  attached 
near  the  center  of  the  fetal  surface  of  the  placenta. 

Condition  of  the  Fetal  Membranes  at  Birth. — The  mem- 
branes discharged  with  the  placenta  at  birth  are,  from  with- 
out inward,  the  decidua  vera,  decidua  reftexa,  chorion  and 
amnion.  The  amniotic  fluid,  in  which  the  fetus  floats, 
reaches  its  maximum  amount  at  about  the  sixth  month.  It 
is  sufficient  then  to  force  the  amnion  closely  against  the  cho- 


356  REPRODUCTION 

rion,  covered  by  the  decidua  reflexa;  these  last  named  (cho- 
rion  and  reflexa)  are  in  turn  forced  everywhere  against  the 
decidua  vera.  The  result  is  that  all  four  become  practically 
one  membrane,  though  the  union  between  amnion  and  cho- 
rion  is  not  so  close  as  that  between  the  other  layers.  These 
membranes  constitute,  then,  a  sac  filled  with  fluid.  The  sac 
is  ruptured  in  labor,  and  the  child  escapes  through  the  rent. 
Afterward  the  decidua  vera  and  placenta  are  detached,  and 
escape  together  as  the  "after  birth." 

Development  of  the  Circulation. — The  development  of  the 
circulation  may  be  considered  in  these  stages :  ( i )  Vitelline 
circulation,  (2)  placental  circulation,  (3)  adult  circulation. 
The  heart  is  the  propelling  organ  in  all  these. 

i.  Vitelline  Circulation. — The  blood  and  the  vessels  make 
their  appearance  almost  as  early  as  the  primitive  groove. 
Certain  blastodermic  cells  are  transformed  into  both  red  and 
white  corpuscles.  They  are  larger  than  the  adult's  cells  and 
both  are  nucleated.  Blastodermic  cells  also  group  to  form 
small  tubes,  which  constitute  the  area  vasculosa.  At  the 
same  time  mesoblastic  cells  develop  two  tubes,  one  along 
each  side  of  the  body,  which  soon  unite  to  form  a  single  one, 
representing  the  heart.  It  becomes  enlarged  and  twisted 
upon  itself,  and  pulsations  begin  in  it  at  a  very  early  date. 
The  heart  is  in  the  median  line  and  gives  off  two  arches 
which  unite  below  to  form  the  abdominal  aorta.  From  the 
arches  pass  branches  to  the  area  vasculosa,  which  now  form 
a  nearly  circular  plexus  around  the  embryo.  Two  of  these 
branches,  larger  than  the  others,  enter  the  umbilical  vesicle 
and  become  the  omphalo-me  sent  eric  arteries;  these  are  two 
corresponding  veins.  This  circulation  through  the  ompha- 
lo-mesenteric  vessels  and  the  area  vasculosa  does  not  con- 
tinue long  in  the  human  being.  As  soon  as  the  allantois 
is  formed  and  the  placental  circulation  begins  to  be  set  up, 
the  omphalo-mesenteric  vessels  are  obliterated  and  the  place 
of  the  first  circulation  is  taken  by  the  second. 

Development  of  the  Heart. — The  tube  just  mentioned  as 


PLACENTAL  CIRCULATION  357 

representing  the  heart  has  communicating  with  it  two  veins 
at  its  lower  extremity  and  two  arteries  at  its  upper.  Soon 
the  tube  becomes  twisted  upon  itself  so  that  the  upper  (ar- 
terial) is  thrown  in  front  of  the  lower  (venous).  The  loop 
is  V-shaped  and  is  the  outline  of  the  future  ventricles.  Af- 
terward a  constriction  forms  the  auricle.  At  this  time  the 
heart  consists  of  a  single  ventricle  and  a  single  auricle. 
Later  the  ventricular  and  auricular  septa  are  formed.  The 
latter  appears  after  the  former  and  is  incomplete ;  the  open- 
ing left  between  the  auricles  is  the  foramen  ovale. 

2.  Placental  Circulation. — As  the  allantois  is  developed 
and  the  vitelline  circulation  is  abolished,  the  hypogastric  ar- 
teries are  given  off  first  from  the  aorta,  but  later  (with  the 
development  of  the  vessels  of  the  lower  extremities)  they 
are  pushed  down,  as  it  werej  so  that  they  take  origin  from 
the  internal  iliacs.  They  pass  to  the  umbilicus  and  thence  to 
the  placenta  by  the  cord.  Blood  is  at  first  returned  from  the 
placenta  by  two  umbilical  veins,  but  these  soon  fuse  into  one. 

Object  of  Placental  Circulation. — Since  the  activity  of  the 
respiratory  and  alimentary  tracts  has  not  been  established, 
their  functions  must  be  performed  by  those  of  the  mother 
and  the  necessary  materials  supplied  from  her  blood.  Con- 
sequently there  must  be  a  continual  passage  of  fetal  blood  to 
and  from  the  placenta  to  discharge  effete  matter  and  to  ab- 
sorb nutriment.  Certain  modifications  of  the  circulatory  ap- 
paratus, not  requisite  after  birth,  are  necessary  to  bring  this 
about. 

Course  of  Fetal  Circulation. — The  umbilical  vein  contain- 
ing blood  enriched  with  oxygen  and  other  materials  enters 
the  body  at  the  umbilicus  and  passes  to  the  under  surface  of 
the  liver.  Hbre  it  divides  into  two  branches.  The  larger 
joins  the  portal  vein  and  enters  the  liver ;  the  smaller  is  the 
ductus  venosus,  which  enters  the  ascending  vena  cava. 

The  ascending  vena  cava,  when  it  enters  the  right  auricle, 
therefore,  contains  blood  from  the  lower  extremities,  blood 
which  has  come  from  the  placenta  directly  through  the 


358 


REPRODUCTION 


ductus  venosus,  and  blood  which  has  come  from  the  pla- 
centa indirectly  through  the  liver.     Considering  that  blood 


FIG.  no. — Diagram  illustrating  the  circulation  through  the  heart  and 
the  principal  vessels  of  a  fetus.     (From  Yeo  after  Cleland.) 

a,  umbilical  vein;  b,  ductus  venosus;  f,  portal  vein;  e,  vessels  to  the  viscera; 
d,  hypogastric  arteries;  c,  ductus  arteriosus. 

from  the  body  of  the  fetus  is  venous  and  that  blood  directly 
from  the  placenta  is  arterial,  the  contents  of  the  ascending 
vena  cava  are  mixed  when  they  enter  the  heart.  The  Eus- 


PLACENTAL  CIRCULATION  359 

tachian  valve,  together  with  the  direction  of  the  entering 
current,  causes  the  blood  from  the  ascending  vena  cava  to 
pass  through  the  foramen  ovale  into  the  left  auricle. 

Blood  from  the  upper  extremities  (impure)  enters  the 
right  auricle  through  the  descending  vena  cava.  The  Eus- 
tachian  valve  and  the  direction  of  the  current  here  again 
cause  this  blood  to  enter  the  right  ventricle.  There  is  sup- 
posed to  be  very  little  mingling  of  blood  from  the  two  venae 
cavae  as  it  passes  thus  through  the  right  auricle.  At  the 
same  time  the  blood  which  has  entered  the  left  auricle 
through  the  foramen  ovale,  augmented  slightly  by  blood 
from  the  ill-developed  pulmonary  veins,  passes  into  the  left 
ventricle.  The  ventricles  now  contract  simultaneously. 

Blood  from  the  right  ventricle  (impure)  passes  in  small 
part  through  the  pulmonary  artery  to  the  lungs,  but  chiefly 
through  a  tube,  the  ductus  arteriosus,  into  the  descending 
part  of  the  aortic  arch. 

Blood  from  the  left  ventricle  (mixed)  enters  the  aorta  and 
goes  to  the  system  at  large. 

The  vessels  going  to  the  head  and  upper  extremities  are 
given  off  from  the  aortic  arch  before  it  is  joined  by  the 
ductus  arteriosus.  Since  the  ductus  arteriosus  contains  im- 
pure blood,  the  supply  going  to  the  upper  extremities  is 
purer  than  that  going  to  the  lower. 

Of  the  blood  which  passes  down  the  aorta  a  part  leaves 
by  the  hypogastric  arteries,  to  go  again  to  the  placenta,  while 
the  other  part  is  distributed  to  the  trunk  and  lower  extremi- 
ties. 

It  thus  appears  that  the  liver  is  the  only  organ  of  the 
fetus  which  receives  pure  blood,  and  that  the  head  and  upper 
extremities  are  better  provided  for  in  this  respect  than  are 
the  lower  parts.  This  may  account  for  the  relatively  large 
liver  of  the  fetus,  and  for  the  fact  that  the  upper  extremities 
are  better  developed  than  the  lower. 

The  ductus  arteriosus,  ductus  venosus,  foramen  ovale, 
Eustachian  valve,  hypogastric  (umbilical)  arteries  and  the 


360  REPRODUCTION 

umbilical  vein  are  the  organs  which  distinguish  the  placental 
circulation,  and  they  all  partially  disappear  after  birth,  as 
will  be  immediately  seen. 

3.  Adult  Circulation. — The  circulation  as  it  exists  in  the 
adult  has  been  described.  It  is  only  necessary  to  see  what 
changes  mark  its  establishment. 

When  the  child  is  born  detachment  of  the  placenta,  or 
ligation  of  the  cord,  stops  the  placental  circulation.  The 
first  noticeable  effect  comes  from  the  consequent  deoxygen- 
ation  of  the  blood.  The  respiratory  center  is  stimulated  and 
the  child  gasps  to  fill  the  hitherto  collapsed  lungs  with  air. 
Owing  to  the  diminished  resistance  in  the  expanded  lungs, 
the  pulmonary  artery  begins  to  carry  most  of  the  blood  from 
the  right  ventricle,  and  the  ductus  arteriosus  commences  to 
atrophy.  .Before  birth,  too,  the  Eustachian  valve  becomes 
less  distinct  and  the  foramen  ovale  partly  closes.  At  labor 
a  kind  of  valve  guards  the  opening  of  the  foramen  ovale  and 
allows  the  escape  possibly  of  a  little  blood  from  the  right 
into  the  left  auricle,  but  none  in  the  opposite  direction.  It 
commonly  closes  about  the  tenth  day  of  extra-uterine  life. 
The  ductus  arteriosus  is  reduced  to  the  condition  of  an  im- 
pervious fibrous  cord  between  the  third  and  tenth  days  after 
birth. 

The  hypogastric  arteries,  umbilical  vein  and  ductus  ven- 
osus  are  closed  between  the  second  and  fourth  days.  That 
part  of  each  hypogastric  artery  between  the  internal  iliac 
and  the  upper  lateral  part  of  the  bladder  remains  in  adult  life 
as  the  superior  vesical  artery;  the  part  between  this  point 
and  the  umbilicus  is  that  which  atrophies.  The  umbilical 
vein  remains  as  the  round  ligament  of  the  liver.  The  ductus 
venosus  is  represented  by  a  fibrous  cord  in  the  fissure  for 
the  ductus  venosus  in  the  liver. 

The  Skeleton. — The  appearance  of  the  notochord  and  of 
the  protovertebrse,  or  somites,  has  been  observed.  The  noto- 
chord becomes  a  thin  line  of  soft  cartilage,  around  which  the 
bodies  of  the  vertebra  are  developed,  though  it  does  not 


NERVOUS   SYSTEM  361 

itself  become  those  bodies.  The  protovertebrse  were  seen  to 
lie  longitudinally  on  either  side  of  the  notochord.  These 
grow  around  the  neural  canal  dorsally  and  the  notochord 
ventrally  to  form  the  vertebrae.  From  them  also  are  de- 
veloped the  muscles  and  skin  of  the  back. 

The  cranium  is  developed  as  a  modification  of  the  verte- 
bral column. 

All  the  bones  are  in  early  fetal  life  cartilaginous  or  mem- 
branous. Centers  of  ossification  appear  at  one  or  more 
points  in  each  bone. 

The  bones  of  the  extremities  are  not  at  first  separate. 
They  bud  out  from  the  upper  and  lower  parts  of  the  trunk, 
to  be  subdivided  later. 

Nervous  System. — The  origin  of  the  nervous  system  has 
been  indicated  in  describing  the  neural  canal.  The  meso- 
dermic  cells  multiply  and  fill  the  tube,  until  only  the  canal  of 
the  spinal  cord  is  left.  Headward  the  neural  canal  termin- 
ates in  a  dilated  extremity,  which  soon  becomes  divided  into 
three  vesicles,  anterior,  middle  and  posterior.  From  these 
are  developed  the  different  parts  of  brain.  Some  of  these 
parts  develop  much  more  rapidly  than  others,  and  we  thus 
account  for  the  predominant  size  of  the  cerebrum.  At  first 
there  are  no  cerebral  convolutions,  but  later  the  cavity  of 
the  cranium  seems  too  small  for  the  brain  and  the  charac- 
teristic infoldings  occur. 

The  eye  is  formed  by  the  projection  of  the  optic  vesicle 
from  the  side  of  the  anterior  brain  vesicle. 

The  internal  ear  is  formed  by  the  projection  of  the  audi- 
tory vesicle  from  the  posterior  brain  vesicle. 

The  alimentary  canal  is  formed  by  being  pinched  off 
from  the  mesodermic  layer  of  splanchnopleure.  It  com- 
municates for  some  time  by  means  of  the  vitelline  duct  with 
the  umbilical  vesicle.  When  cut  off  from  the  latter  it  is  a 
straight  tube,  occupying  the  long  axis  of  the  body  just  in 
front  of  the  vertebral  column,  and  is  divided  into  the  fore- 
gut,  hindgut  and  a  central  part.  Later  it  communicates  above 


362  REPRODUCTION 

with  the  pharynx  and  mouth  and  opens  below  upon  the  ex- 
ternal body  surface  (anus).  The  liver  and  pancreas  are  de- 
veloped from  protrusions  from  the  sides  of  the  duodenum. 

The  bladder  has  been  seen  to  be  that  part  of  the  allantois 
which  is  constricted  off  and  remains  in  the  body. 

The  lungs  are  developed  from  the  esophagus  and  at  first 
lie  in  the  abdominal  cavity;  but  the  formation  of  the  dia- 
phragm fixes  them  in  the  thorax. 

The  kidneys  are  developed  from  the  Wolffian  bodies. 
These  bodies  are  'embryonic  structures  only.  Each  is  a  tube 
lying  parallel  to  the  vertebral  column  on  either  side  of  it. 
This  tube  consists  of  a  collection  of  tubules,  which  unite  to 
form  a  common  excretory  duct.  This  duct  joins  the  corres- 
ponding one  from  the  opposite  side  to  empty  into  the  alimen- 
tary canal  opposite  the  allontoic  stalk.  Outside  the  Wolffian 
bodies  are  two  other  ducts,  the  ducts  of  Miiller.  They  also 
enter  the  intestine. 

The  WolfBan  body  finally  gives  place  to  the  kidney,  from 
which  the  ureter  is  developed. 

In  the  female  the  ducts  of  Miiller  become  the  tube,  uterus 
and  vagina.  In  the  male  they  atrophy. 

Just  behind  the  Wolffian  bodies  are  developed  the  ovaries 
or  the  testes,  as  the  case  may  be. 


The  development  of  a  few  of  the  organs  has  thus  been 
simply  referred  to. 

Satisfactory  explanation  of  these  procedures  can  be  given 
only  in  extended  works  on  embryology,  and  this  section  may 
be  closed  with  the  subjoined  table  of  development,  which  is 
abbreviated  from  one  by  Heisler: 

First  Week. — Segmentation  and  passage  of  ovum  to 
uterus. 

Second  Week. — Ovum  in  uterus.  Decidua  reflexa  present. 
Entoderm  and  ectoderm  layers  formed — also  mesoderm. 
Embryonal  area,  primitive  streak  in  primitive  groove.  Cho- 


FETAL  DEVELOPMENT  363 

rion  and  villi.  Amnion  folds.  Umbilical  vesicle  partly 
formed.  Vascular  area.  Two  primitive  heart  tubes.  Gut 
tract  partly  formed. 

Third  Week. — Body  indicated.  Dorsal  outline  concave. 
Vitelline  duct.  Amnion.  Allantoic  stalk.  Visceral  arches. 
Heart  divides.  Vitelline  circulation  begins.  Gut  tract  still 
connected  with  umbilical  vesicle.  Liver  evagination  begins. 
Anal  plate.  Pulmonary  protrusion.  Wolffian  bodies.  Neu- 
ral canal.  The  brain  vesicles.  Optic  and  otic  vesicles.  Ol- 
factory plates.  Notochord. 

Fourth  Week. — Flexion  of  body.  Yolk-sac  largest  size. 
Somites  well  formed.  Allantois  grows.  Vitelline  circulation 
complete.  Allantoic  vessels  developing.  Pharynx,  esopha- 
gus, stomach  and  intestine  differentiated.  Pancreas  begins. 
Pulmonary  protrusion  bifurcates.  Ventral  roots  of  spinal 
nerves.  Limb  buds  apparent. 

Fifth  Week. — Umbilical  vesicle  begins  to  shrink.  Cord 
longer  and  spiral.  Length  of  fetus  two-fifths  of  an  inch. 
Primitive  aorta  divides  into  aorta  and  pulmonary  artery. 
Intestine  shows  loops.  Bronchi  divided.  Ducts  of  Miiller. 
Epidermis.  Olfactory  lobe.  Eyes  move  forward.  Limb 
buds  segment.  Digitation  indicated. 

Sixth  Week. — Umbilical  vesicle  shrunken.  Amnion 
larger.  Vitelline  circulation  supplanted  by  allantoic^  Teeth 
indicated.  Duodenum,  cecum.  Rectum.  Larynx.  Genital 
folds  and  ridges.  Dorsal  roots  of  spinal  nerves.  Eye-lids. 
Lower  jaw  and  clavicle  begin  to  ossify.  Vertebrae  and  ribs 
cartilaginous.  Fingers  separate. 

Seventh  Week. — Body  and  limbs  well  defined.  Heart 
septa  complete.  Transverse  and  descending  colon.  Nails 
indicated.  Cerebellum  indicated.  Muscles  recognizable. 
Ossification  in  cranium  and  vertebrae  begins. 

Eighth  Week. — Hiead  somewhat  elevated.  Parotid  gland. 
Gall  bladder.  Miillerian  ducts  unite.  Genital  groove.  Mam- 
mary glands  begin.  Sympathetic  nerves.  Nose  discernible. 
Additional  centers  of  ossification. 


364  REPRODUCTION 

Ninth  Week.-^Weight,  three-fourths  of  an  ounce. 
Length,  one  and  a  quarter  inches.  Pericardium.  Anal  ca- 
nal. External  genitals  begin  to  indicate  sex.  Ovary  and 
testis  distinguishable.  Kidney  characteristic.  External  ear 
indicated. 

Third  Month. — Weight,  four  ounces.  Length,  two  and 
three-quarter  inches.  Chorion  frondosum.  Placental  ves- 
sels. Tonsil.  Stomach  rotates.  Vermiform  appendix. 
Liver  large.  Epiglottis.  Ovaries  descend.  Testes  in  false 
pelvis.  Hair  and  nails.  Development  of  different  parts  of 
brain.  Limbs  have  definite  shape. 

Fourth  Month. — Weight,  seven  and  three-quarter  ounces. 
Length,  five  inches.  Head  one-fourth  of  entire  body.  Germs 
of  permanent  teeth.  Distinction  of  external  genitals  well 
marked.  Spinal  cord  ends  at  end  of  coccyx.  Eye-lids  and 
nostrils  closed. 

Sixth  Month. — Weight,  two  pounds.  Length,  twelve 
inches,  Amnion  at  maximum  size.  Trypsin  in  pancreatic  se- 
cretion. Air  vesicles.  Eye-lashes.  Lobule  of  ear  charac- 
teristic. 

Seventh  Month. — Weight,  three  pounds.  Length,  four- 
teen inches.  Meconium.  Ascending  colon.  Testes  at  inter- 
nal rings.  Cerebral  convolutions  evident.  Differentiation 
of  muscular  tissue. 

Eighth  Month. — Weight,  four  to  five  pounds.  Length, 
sixteen  inches.  Body  more  plump.  Ascending  colon  larger. 
Testes  in  inguinal  canal.  Skin  brighter  color.  Nails  pro- 
ject beyond  finger  tips. 

Ninth  Month. — Weight,  six  to  seven  pounds.  Length, 
twenty  inches.  Meconium  dark  green.  Testes  in  scrotum. 
Labia  majora  in  contact.  Spinal  cord  ends  at  last  lumbar 
vertebra.  Ossification  centers  completed. 


INDEX 


Abducens  nerve,  286 
Absorption,  from  stomach,  81 

from  intestines,  96 
Accommodation,  ocular,  315 
Adipose  tissue,  14 

formation  of,  178 

value  of,  178,  179 
Adrenal  glands,  33 
Afferent  nerves,  234 
Air,  amount  necessary,  161 

alterations  of,  in  lungs,  150 

cells,  137 

composition  of,  148 

diffusion  of,  in  lungs,  148 

vesicles,  137 
Albuminoids,  175 
Allantois,  349 
Alveoli,  capacity  of,  147 
Ammonia     compounds      antece- 
dents of  urea,  203,  204 
Amnion,  the  347 
Amniotic  cavity,  348 
Amylopsin,  102 
Anabolism,  172 
Animal  heat,  184 

loss  of,  by  evaporation,  189 

radiation  of,  189 

relation  of,  to  force,  185 

source  of,  185 

specific,  187 

total,  187 
Antehelix,  319 
Anterior  chamber  of  eye,  313 

elastic  lamella,  311 

fundamental   fasciculus,  241 

radicular  zone,  241 
Aphasia,  270 
Aqueous  humor,  314 
Arachnoid,  236 
Archenteron,  340 
Arteria  centralis  retinae,  313,  314 
Arterial  circulation,  47 

effect  of  respiration  on,  164 


Arteries,  47 

elasticity  of,  48 
Arytenoid  cartilages,  133,  325 
Asphyxia,  162 
Auditory  canal,  external,  319 

center,  325 
Auditory  nerves,  289 

terminations  of,  322 
Auerbach,  plexus  of,  97 
Auricle,  left,  44 

right,  44 
Axis  cylinders,  222 

Bacteria  in  digestion,  120 
Bartholin's  duct,  72 
Bellini,  straight  tubes  of,  197 
Bertin,  columns  of,  195 
Bile  ducts,  108 
Bile,  in  digestion,  105 

functions  of,  115 

properties    and    composition 

of,  no 

Bilirubin,  in 
Binocular  vision,  317 
Bladder,  207 

absorption  of,  207 

structure  of,  207 
Blastoderm,  340 
Blood,  the,  35 

alterations  of,  in  lungs,  157 

amount  in  body,  35 

arterialization  of,  150 

coagulation  of,  40 

color  of,  35 

composition  of,  36 

functions  of,  35 

plasma,   36 

platelets,  39 

serum,  36 
Bone,  16 

Bone,  marrow,  18 
Bowman's  capsule,  196 
Brain,  the,  251 


365 


366 


INDEX 


membranes  of,  236 
Breathing  (see  Respiration,  131) 
Broca's  convolution,  270 
Bronchi,  136 

capacity  of,  148 
Bronchioles,  137 
Burdach,  columns  of,  247 

Capillaries,  the,  48 
Capillary,  importance,  50 
Carbohydrates,  176 

final  products  of,  176 

value  of,  in  nutrition,  176 
Carbon,  amount  in  excreta,  182 

dioxide,  amount  exhaled,  154 
amount  in  bipod,  157 
condition  of,  in  blood,  155 
discharge,  151 
gain  of,  in  lungs,  151 
inhalation  of,  161 
interchange    of,    in    lungs, 

156 
source  of,  exhaled,  154 

monoxide,  inhalation  of,  161 
Cardiac  cycle,  43 

length  of,  43 
Cartilage,  15 

Hyaline,  15 

white  fibrous,  15,  16 

yellow  elastic,  15 
Cauda  equina,  238 
Cecum,  117 
Celenteron,  340 
Celom,  344 
Cell,  i 

properties  of,  3 

structure  of,  2 
Centrifugal  nerves,  230 
Centripetal  nerves,  230 
Cerebellum,  the,  274 

anatomy  of,  274 

fibers  of,  274 

function  of,  275 

peduncles  of,  274 
Cerebral  localization,  268 
Cerebro-spinal  axis,  235 

system,  216 
Cerebrum,  the,  260 

cells  of,  263 


Cerebrum,   convolutions   of,  262, 
263 

fibers  of,  263 

fissures  of,  260 

functions  of,  271 

lobes  of,  260 

motor  centers  in,  268 
paths  from,  268 

sensory  centers  in,  269 
paths  to,  269,  270 

special  centers  in,  269 
Cerumen,  30 
Cervical  ganglia,  299 
Cholesterin,  in 
Chorda  dorsalis,  344 

tympani,  288 
Chordal  folds,  345 

plates,  344 
Chorion,  351 
Choroid  coat  of  eye,  311 
Chyme,  100 
Ciliary  muscle,  312 

processes,  312 
Circulation,  the,  41 

pulmonic,  41 

systemic,  41 

Circumvallate  papillae,  318 
Claustrum,  258 
Cochlea,  322,  323 
Colloids,  123 
Colon,  117 
Colostrum,  31 
Common  bile  duct,  109 
Complemental  air,  147 
Conjunctiva,  311 
Connective  tissue,  n 
Convoluted  tubules,  197 
Cornea,  311 
Corona  radiata,  258 
Corpora  quadrigemina,  259 

striata,  256,  258 
Corpus  luteum,  332 
Corti,  rods  of,  323 
Coughing,  145 
Cranial  nerves,  276 
Creatin,  205 
Cricoid  cartilage,  133. 
Crossed  pyramidal  tracts,  245 
Crura  cerebri,  256 


INDEX 


367 


Crystalloids,  123 
Cutaneous  respiration,  160 

sensations,  center  for,  269 
Cutis  vera,  210 
Cystic  duct,  109 

Death,  172 

Decidua  menstrualis,  352 

of  pregnancy,  352 
Defecation,  121 
Deglutition,  78 

mechanism  of,  79 

nervous  control  of,  80 
Dendrites,  216,  222 
Descemet,  membrane  of,  311 
Descendens  hypoglossi,  295 
Diet,  amount  of,  183 

determination  of,  181 

necessary  constituents  of,  182 
Dietetics,  181 
Diffusion  in  lungs,  148 
Digestion,  68 

gastric,  81 

intestinal,  96 

object  of,  68 

processes  in,  70 
Direct  cerebellar  tract,  241 
Discus  proligerus,  331 
Dreams,  302 
Ductus  arteriosus,  359 

communis  choledochus,  109 

venosus,  359 
Dura  mater,  236 
Dyspnea,  162 

Ear,  the,  318 

drum,  320 

external,  319 

internal,  320 

middle,  320 
Ectoderm,  339 
Efferent  nerves,  231 
Eighth  nerve  (see  Auditory,  289) 
Elasticity  of  arteries,  48 
Electrical  stimulation  of  nerves, 

234 
Elementary  tissues,  7 

derivation  of,  7 

varieties  of,  8 


Eleventh  nerve  (see  Spinal  acces- 
sory, 294) 

Embryonal  area,  341 
Encephalon  (see  Brain,  251) 
Endocardium,  42 
Entoderm,  340 
Enzymes,  69 

characteristics  of,  69 

classification  of,  69 

manner  of  action  of,  70 
Epiblast,  339 
Epidermis,  208 
Epiglottis,  135 
Epinephrine,  34 
Epithelial  tissue,  7 

ciliated,  9 

columnar,  9 

glandular,  II 

modified,  9 

neuro,  n 

squamous,  8 

stratified,  8 

varieties  of,  8 
Esophagus,  79 
Eupenea,  162 
Eustachian  valve,  359 
Excretion,  192 
Expiration,  143 

causes  of,  143 

forced,  144 

effect  of,  in  blood  pressure, 

166 

Expired  air,  composition  of,  151 
External  capsule,  258 
Eye-ball,  anatomy  of,  311 

movements  of,  309 

protection  of,  308 

Facial  nerve,  286 
Fats,  as  foods,  66 

end  products  of,  177 
Fauces,  78 

Feces,  composition  of,  120 
Fecundation,  337 
Ferrein,  pyramids  of,  195 
Fertilization,  337 
Fetal  membranes,  347 
Fibrin,  40 
Fibrous  tissue,  13 


368 


INDEX 


Fifth  nerve  (see  Trifacial,  281) 
Filum  terminate,  238 
First  nerve  (see  Olfactory,  276) 
Foods,  63 

classification  of,  64 

fate  of  in  body,  173 

how  absorbed,  128 

potential  energy  of,  185 

where  absorbed,  128 
Fourth    nerve     (see    Patheticus, 
280) 

ventricle,  253 
Fovea  centralis,  313,  314 
Fungiform  papillae,  318 
Funiculi  graciles,  252 

of  Rolando,  252 

Gall  bladder,  109 
Gases  in  intestine,  127 
Gastric  glands,  cells  of,  85 
nerve  supply  of,  89 
structure  of,  85 
varieties  of,  85 

juices,  action  of  on  foods,  92 
properties  and  composition 

of,  90 

secretion  of,  87 
Gastrula,  340 
Glands,  27 

adrenal,  33 

agminate,  100 

intestinal,  96 

gastric,  85 

mammary,  31 

of  Brunner,  99 

of  Lieberkuhn,  99 

parotid,  72 

salivary,  71 

sebaceous,  30 

secretion  in,  29 

solitary,  TOO 

sublingual,  72 

submaxillary,  72 

sweat,  210 

thyroid,  32 
Glisson's  capsule,  105 
Glomeruli,  renal,  195 
Glosso-pharyngeal  nerve,  289 
Glycocholic  acid,  in 


Glycogen,  formation  of  in  liver, 

112 

Golgi,  corpuscles  of,  230 
Goll,  column  of,  247 
Gustatory  center,  269 
Graafian  follicles,  330 

Hairs,  210 
Haversian  canals,  17 

systems,  18 
Hawking,  145 
Hearing,  sense  of,  318 
Heart,  anatomy  of,  42 

beats  of,  43 

contractions  of,  43 

development  of,  356 

diastole  of,  43 

innervation  of,  46 

sounds  of,  46 

systole  of,  43 

valves  and  openings  of,  44 

work  of,  45 
Heat  of  body  (see  Animal  heat, 

184) 

Hemoglobin,  38 
Henle,  loops  of,  197 

sheath  of,  220 
Hepatic  artery,  105 
Hiccough,  145 
Hippuric  acid,  205 
Hunger,  seat  of,  64 
Hydrochloric  acid,  90 
Hydrogen,  inhalation  of,  161 
Hyperopia,  316 
Hyperpnea,  162 
Hypoblast,  339 
Hypoglossal  nerve,  295 
Hypoxanthin,  205 

Ileo-cecal  valve,  118 
Impregnation,  337 
Incus,  320 
Infundibula,  137 
Innervation  of  vessels,  50,  51 
Inspiration,  141 

causes  of,  142 

effects  of  on  blood  pressure. 
164 

muscles  of,  143 


INDEX 


369 


Inspired  air,  composition  of,  151 
Interlobular  veins,  106 
Internal  capsule,  259 

respiration,  159 
Intestinal  glands,  99 
Intestine,  digestion  in,  96 

divisions  of,  97 

movements  of,  121 

nerve  supply  of,  117 

structure  of,  119 
Intralobular  veins,  106 
Intrapuimonary  pressure,  146 
Intrathoracic  pressure,   146 
Iris,  the,  312 
Katabolism,  172 
Kidney,  blood  supply  of,  199 

structure  of,  192 
Kinetic  energy,  186 
Krause,  end  bulbs  of,  228 

Labyrinth,  bony,  321 

membranous,  322 
Lacrymal  apparatus,  309 

duct,  309 

glands,  309 

sac,  309 

Lactates,  discharge  of,  205 
Lacteals,  98 
Large  intestine,  117 

digestive  changes  in,  119 

divisions  of,  117 

movements  of,  121 

structure  of,  119 
Larynx,  133,  325 

nerve  supply  of,  327 
Laughing,  145 
Lenticular  ganglion,  284 
Leukocytes     (see      White     cor- 

T  •  u    i  t»scles>  39) 
Lieberkuhn,  crypts  of,  99 
Liquor  amnii,  348 

sanguinis  (see  Blood  plasma 

36) 
Liver,  anatomy  of,  104 

histology  of,  108 

lymphatics  of,  no 

nerve  supply  of,  no 

vessels  of,  105 
Lumbar  ganglia,  299 

24 


Lungs,  138 

capacity  of,  147 
Lymphatic  glands,  59 
Lymph,  57 

course  of,  57 

flow  of,  61 

properties    and    composition 

of,  59 
Lymph  vessels,  origin  of,  57 

Macula  lutea,  313 
Malleus,  320 
Malpighian  bodies,  195 

pyramids,  195 
Mammary  glands,  31 
Mastication,  71 
Maturation,  32 
Meckel's  ganglion,  284 
Medulla  oblpngata,  251 
centers  in,  254,  255 
functions  of,  254 
gray  matter  of,  253 
pyramids  of,  251 
relation  of  cord  tracts  to,  253 
white  fibers  of,  253 
Meissner,  corpuscles  of,  229 

plexus  of,  97 
Membrana  tympani,  320 
Menstruation,  335 
Mesoblast,  340 
Mesoderm,  341 
Metabolism,  172 

conditions  influencing,  179 
Micturition,  208 

center  for,  208 
Milk,  human,  32 
Mitral  valve,  44 
Mixed  lateral  column,  240,  242 
Motor  oculi  communis,  278 

paths  from  cerebrum,  266 
Muscular  contractions,  23 

physiological  characteristics, 

23 

tissues,  19 
Myopia,  316 

Nails,  the,  210 
Nasal  duct,  309 


370 


INDEX 


Nerve  cells,  221 
centers,  221 
fibers,  217 
action  of  electricity  upon, 

234 

afferent,  232 
classification  of,  230 
degeneration  of,  239 
directions   of   currents   in, 

233 

efferent,  231 
individuality  of,  221 
medullated,  217 
non-medullated,  219 
properties  of,  230 
speed  of  conduction  in,  234 
terminals,  225 
between  epithelial  cells,  227 
in  bulbs  of  Krause,  228 
in    Golgi's   corpuscles,   230 
in  glands,  226 
in  hair-follicles,  226 
in    Meissner's    corpuscles, 

229 

in  Pacinian  corpuscles,  227 
in  plain  muscle,  226 
Nerve  terminals,  in  striped  mus- 
cle, 225 

in  tactile  menisques,  229 
trunks,  219 

Nervous  system,  the,  214 
development  of,  361 
divisions  of,  216 
general  functions  of,  214 
Neural  canal,  343 
Neuroglia,  216 
Neurons,  216 

communication  between,  223 
Ninth  nerve  (see  Glosso-pharyn- 

geal,  289) 

Nitrogen,  amount  necessary,  182 
Nitrogenous  equilibrium,  174 
Nitrous  oxide,  inhalation  of,  161 
Nutrition,  171 

Olfactory  bulb,  277 
cells,  276 
center,  269 
nerve,  276 


Olivary  bodies,  251 
Omphalo-mesenteric  vessels,  356 
Ophthalmic  ganglion,  284 
Optic  center,  269 

commissure,  277 

nerve,  277 

thalami,  258 

functions  of,  258 

tracts,  277 
Osmosis,  123 
Otic  ganglion,  284 
Ova,  329 

Ovary,  secretion  of,  34 
Ovulation,  334 
Oxidation  in  the  body,  171 
Oxygen,  amount  consumed,  154 

amount  in  blood,  157 

condition  of  in  blood,  157 

entrance  of  into  tissues,  159 

loss  of  in  lungs,  148 

Pacini,  corpuscles  of,  227 
Pancreas,   anatomy  of,   100 

histology  of,   100 

internal  secretion  of,  103 

nerve  supply  of,  103 

secretion  in,  103 
Pancreatic  juice,  101 
Partial  pressure  of  gases,  156 
Patheticus  nerve,  280 
Pepsin,  91 
Peptones,  92 
Pericardium,  42 
Periosteum,  18 
Perspiration,  212 
.Pharynx,  132 
Pia  mater,  237 
Pinna,  319 
Pituitary  body,  34 
Placenta,  354 
Placental  circulation,  357 
Pneumogastric.  nerve,  290 

influence    of   on    respiration, 

168 
stomach     and     intestines, 

168 
Pons  Varolii,  255 

functions  of,  255 
Posterior  chamber  of  eye,  313 


INDEX 


371 


Prehension,  70 
Presbyopia,  316 
Pronucleus,  female,  334 

male,  337 
Proteids  as  foods,  67 

circulating,  174 

final  products  of,  174 

tissue,  174 
Proteoses,  92 
Protovertebrse,  345 
Ptyalin,  74 
Pulse,  the,  53 
Pupil,  the,  313 

Ranvier,  nodes  of,  218 
Reaction  of  pupil,  316 
Receptaculum  chyli,  57 
Rectum,  118 
Red  corpuscles,  37 
Reflex  action,  248 
Refraction,  ocular,  314 
Reil,  island  of,  263 
Renal  tubules,  197 
Rennin,  91 
Reproduction,  328 
Reserve  air,  147 
Residual  air,  147 
Respiration,  131 

abnormal,  161 

afferent  nerves  of,  167 

center  for,  167 

costal,  146 

cutaneous,  160 
Respiration,    diaphragmatic,    146 

effect  of,  on  blood  pressure, 

164 
,  Efferent  nerves  of,  170 

external,  132 

internal,  131,  158 

mechanism  of,  139 

modified,  145 

nervous  control  of,  166 

object  of,  131 

organs  of,  132 

rate  of,  145 

rhythm  of,  144 

sounds   of,   145 

types  of,  146 
Restiform"  "bodies,  251 


Retina,  313 

Rolando,  fissures  of,  260 

Saliva,  functions  of,  71 

properties    and    composition 

of,  74 
Salivary  glands,  71 

histology  of,  72 

nerve  supply  of,  74 

secretion  in,  76 
Salts,  65 
Schwann,  sheath  of,  217 

white  substance  of,  218 
Sclerotic  coat  of  eyes,  311 
Sebaceous  glands,  30 
Second  nerve   {see   Optic  nerve, 

.  277) 
Secretion,  27 

external,  29 

internal,  29 

paralytic,  76 
Segmentation,  338 

cavity    338 

nucleus,  338 

Semicircular  canals,  322 
Semilunar  valves,  44 
Sensations,  common,  305 

special,  306 
Senses,  the,  305 
Serum-albumin,  36 

-globulin,  36 

Seventh  nerve  (see  Facial,  286) 
Sighing,  145 
Sight,  sense  of,  308 
Sigmoid  flexure,  117 
Sixth  nerve  (see  Ab  due  ens,  286) 
Skin,  excretion  of,  208 

functions  of,  208 

structure  of,  210 
Sleep.  301 

vascular  phenomena  of,  302 
Smegma,  30 
Smell,  sense  of,  307 
Sneezing,  145 
Snoring,  145 
Sobbing,  145 
Sodium  salts,  65 

functions   of,  65 
Solar  plexus,  299 


372 


INDEX 


Somatopleure,  341 
Somites,  345 
Speech,  325,  327 
Spermatozoa,  328 
Spheno-palatine  ganglion,  284 
Spinal  accessory  nerve,  294 
cord,  238 

columns  of,  240 
commissures  of,  238 
cross  section  of,  238 
degeneration  in,  240 
functions  of,  248 
gray  matter  in,  238 
motor  paths  in,  243 
sensory  paths  in,  245 
special  centers  in,  251 
nerves,  296 

Splanchnic  nerves,  299 
Splanchnopleure,  34* 
Starvation,  effects  of,  180 
Steapsin,  102 
Stenson's  duct,  72 
Stercorin,  in 
Stomach,  the,  81 
histology  of,  83 
movements  of,  94 
nervous  supply  of,  96 
Straight  tubules  (renal),  197 
Striated  muscle,  19 

characteristics  of,  19 
Stapes,  the,  320 
Sublobular  veins,  106 
Submaxillary  ganglion,  284 
Succus  entericus,  116 
Supplemental  air,  147 
Suspensory  ligament,  314 
Sweat  glands,  210 
Sweat,    properties,    composition 

of,  212 

secretion  of,  212 
Sylvius,  aqueduct  of,  253 

fissures  of,  267      • 
Sympathetic  system,  216 
Syntonin,  92 

Tactile  sensibility,  306 

acuteness  of,  307 
Taste  beakers,  318 

sense  of,  3J7 


Taurocholic  acid,  in 
Temperature  impressions,  307 

of  body,  184 
Tenon,  capsule  of,  309 
Tenth  nerve  (see  Pneumo gastric, 

290) 

Testes,  secretion  of,  34 
Thermogenesis,  188 
Thermotaxis,  190 
Third    nerve    (see    Motor    oculi 

communis,  278) 
Thirst,  seat  of,  64 
Thoracic  duct,  57 

ganglia,  299 
Thorax,  139. 

Thyroid  cartilage,  133 
gland,  32 

Tidal  air,  147 

Touch,  sense  of,  305,  306 

Trachea,  135 

Tragus,  319 

Tricuspid  valve,  44 

Trifacial  nerve,  281 

Trigeminal  nerve,  281 

Trypsin,  102 

Turck,  column  of,  245 

Twelfth  nerve  (see  Hypoglossal, 

295) 
Tympanum,  320 

Umbilical  cord,  355 

vesicle,  347 
Urea,  202 

daily  discharge  of,  203 

formation  of,  203 
Ureters,  206 
Uric  acid,  204 

daily  discharge  of,  204 
Urine,  constituents  of,  202 

discharge  of,  206 

properties  of,  202 

salts  of,  206 

secretion  of,  199 

variations  in  amount  of,  200 

in  composition  of,  206 
Uriniferous  tubules,  197 

secretory  changes  in,  202 
Vaginal  plexus,  105 


INDEX 


373 


Vagus  nerve  (see  Pneumo gastric, 

290) 

Valvulse  conniventes,  97 
VasQ-motor  nerves,  51 

centers  for,  51 
Vater,  corpuscles  of,  227 
Veins,  48 

valves  of,  50 
Ventilation,  160 
Ventricle,  left,  44 

right,  44 

Vermiform  appendix,  118 
Vestibule  of  ear,  321 
Villi,  98 
Vitelline,  circulation,  356 

duct,  345  ' 
Vitreous  humor,  314 


Vocal  cords,  133,  325 

sounds,  varieties  of,  326 
Voice,  production  of,  325 

Water,  65 

elimination  of  by  kidney,  201 
Wharton's  duct,  72 
White  corpuscles,  39 
Wirsung,  duct  of,  100 
Wolffian  bodies,  362 

Xanthin,  discharge  of,  205 
Yawning,  145 
Zymogen,  101 


MOLOGY 


